Other than visual style, the only other big difference between then and now is that current cartoons operate with the kind of post-modern self-awareness that was still in its infancy 30 years ago. A carbon copy of the original ThunderCats show would be a hard sell in a zeitgiest that can't resist tempering sincerity with irony. Warner Bros. Animation also already tried a more faithful approach back in 2011. It was a fantastic show... that nobody watched.
There's an alternate timeline out there in which the first reboot enjoyed as much success as the rebooted Voltron series has on Netflix. But, we're not living in that timeline, and, in the wake of its previous failure, it's understandable that the company felt a radical shift was worth trying out to maintain ThunderCats' relevancy.
Listen, if you're married to the original ThunderCats and won't accept any substitutes, you just can drown your sorrows by revisiting it. If you worship at the alter of strapping, animated men with countable abs who are relics of the '80s, Dragon Ball is still on the air. But, if you've been keeping up with the critically acclaimed "CalArts" class of 2010 onwards, you'll know that the 'Cats are probably in good hands, and should be content that their legacy is being kept alive for a whole new litter.
ThunderCats Roar debuts on Cartoon Network in 2019.