This is one thing they did that I really don’t like. Some of my battling/competitive Pokémon have moves from previous generations and I would prefer for them to know the moves. For instance I have a Chansey with Toxic but I also have some harder to obtain ones like Clefable knowing Soft-Boiled (Emerald & FRLG tutor) and Zapdos knowing both Twister (Pt & HGSS tutor) and Extrasensory (XD exclusive move). I’ve been enjoying making some unique movesets (whether they are viable is a different story) that require transfer and it looks like those days may be over for me.
I think that's the very point of the move resets. It serves two purposes: to rebalance the game with each generation, and to prevent someone who's been playing longer to get an edge. Your example of Toxic, for instance, is part of the intent of the former: it really WAS a move whose usage was getting out of control, and to curb it, they severely restricted who is allowed to use it in Generation VIII. (Consider it a nerf.) Same goes for Tailwind. For the latter, it's also the same reason Mythical Pokémon are banned in all Ranked modes, and nearly all Online Competitions: so everyone is on the same playing field within a generation, and so you can't just get an advantage by being at the right place at the right time.
One Pokémon that would be a MAJOR problem without a move reset is Machamp. If you could transfer a Machamp forward indefinitely and keep its moves, you could get one with No Guard that knows Fissure, obtained from
Pokémon Red and Blue on the Wii U's Virtual Console. A Pokémon with a one-hit KO move whose Ability boosts it to 100% accuracy would be troublesome in every way. Currently, Umbreon is seeing high usage in
Scarlet and Violet Ranked, in both single and double battles; we don't need it to be out of control by allowing it to have Toxic. A likely future example will be Flutter Mane, as it's a Pokémon that needs nerfing, but its stats and type combination can't be adjusted--an easier way to nerf it in the future is to reduce the moves it can know. (Or nerf Protosynthesis, but that might bring down the less-used ones too like Sandy Shocks and Slither Wing.)