Throughout the World Coronation Series, Ash hasn't used any of his old Pokémon save for Pikachu. Besides this, he's relied almost exclusively on the five Pokémon he caught throughout the Pokémon Journeys anime; Dragonite, Gengar, Lucario, Sirfetch'd, and Dracovish. It would be one thing if he were competing in a regional league, but the WCS pits him against the strongest Trainers from around the world. If Ash really wanted to win, he probably should have needed or felt the need to use some of his older, more experienced Pokémon. At least, that's what many fans believe.
It's possible that Ash's current team is rightfully strong enough to battle the world's strongest trainers. Ash should know his own Pokémon better than anyone, so maybe he sees something in his current team that fans are missing. For everything he's been through with his new team, it's possible he believes that they're just as qualified as any of his other Pokémon to win the WCS.
At the very least, Ash should have had all the time he needs to train up his new Pokémon to become strong. It's hard to say just how long the events of Journeys have been going on in-universe, but it seems safe to say that Ash is currently in the series' equivalent to a Pokémon League Conference. If this is true, then his new Pokémon should be at least around the same level as most of his older Pokémon.
Generally, Ash stops traveling with the team he used for a regional Pokémon League after the League Conference is finished. The rare exceptions to this are Pikachu, Phanpy, Charizard, and whatever Pokémon he took with him from Kanto into the Orange Islands and Johto. He also used to use old Pokémon for special occasions like League Conferences or Frontier Brain battles, but he hasn't done this since Diamond and Pearl. Unless they've been doing some kind of special training off-screen, most of them shouldn't be much stronger than they were when Ash left them. Thus, if the Masters Eight Tournament is really comparable to a League Conference, Ash's new team should be at least as strong as most of Ash's old Pokémon. Since the Masters Eight Tournament is arguably even harder than a Champion League, they might even be at a slightly higher level.
The most reasonable exceptions to this difference in level would be some of Ash's strongest Pokémon. In most regions, Ash was able to train at least one of his Pokémon to be the strongest on his then-current team; most of them were arguably even better than Pikachu. Some popular examples include Charizard, Infernape, and Greninja. Lucario seems to be the one filling this role on Ash's Journeys team, though it was proven to be weaker than Greninja through a battle. Ace Pokémon like these should not have been reasonably surpassed by any member of Ash's latest team. They're the ones that really make fans wonder why Ash isn't altering his team for the Masters Eight tournament.
With that said, Ash still has plenty of reasons to have faith in his current team. For one thing, all of the Pokémon he's using are objectively powerful even by the game's standards. Many Trainers would be happy to have Pokémon like Dragonite, Gengar, and Dracovish on their team. If they could also have a Mega Lucario or Sirfetch'd as good in battle as Ash's, that would be nice, too. This is to say nothing for his Pikachu, who has the most experience of any of his Pokémon by far. If Ash ever does decide to start using old Pokémon again, any member of his current team would make an ideal choice.
Speaking of not using old Pokémon, that's something else worth considering about Ash's approach to the Masters Eight tournament. As stated before, Ash stopped using his older Pokémon for Leagues and competitions after Diamond and Pearl. In all likelihood, he's sticking with his latest team not because they're necessarily his best but because that's just how he's been written for years now.
The problem with Ash being written like this in Journeys, in particular, has to do with the apparent challenge of the WCS. This competition is nothing like the beginner-friendly regional Pokémon League challenges; top-tier Trainers from around the world are competing in ranked battles for the number one spot, including Champions. Ash catching new Pokémon and using them to win in this scenario shouldn't work out as well as it has; it's a conflict between how the story is being written and what the fans believe to be the best logical course of action. As unreasonable as it sounds, Ash will likely continue to rely on his new Pokémon right up until his last major battle of Journeys.
With all that said, Ash's new team might be strong precisely because of how difficult the WCS is. The boy has been consistently using his new Pokémon in ranked battles against objectively powerful trainers. Thus, they have all been regularly subjected to more intense battles than most of his other Pokémon have ever been put through, especially early on. It's reminiscent of how, in the games, players will send out their new, low-leveled Pokémon against higher-leveled Pokémon to gain more experience and level up faster. If this sort of trial by fire doesn't make a Pokémon into a strong and reliable battler, nothing will.
It's also worth noting the ongoing event that involves the distribution of Ash's current team. Between the months of August and September, players of Pokémon Sword and Shield can use distributed serial codes to receive each of the Pokémon on Ash's current team, including Pikachu. According to this event, all of these Pokémon are currently at Level 80; for comparison, none of Ash's Pokémon given out as events have ever been higher than Level 50, not even Pikachu or Charizard. Whatever level the rest of Ash's Pokémon are meant to be, this could arguably be taken as proof that his current team is, in fact, high-leveled in its own right.
Whatever Ash's optimal team is, he has every right to believe that his current team is strong enough to win the Masters Eight tournament. They're all well-trained, highly experienced, and powerful Pokémon in general. He'd almost certainly have an easier time if he used Pokémon like Charizard or Greninja, but the way things are going, he might not have to.