The reason Twice has lasted as long as they have is because for JYP they have to. JYP is a company that has proven they can't make a hit girl group every generation like the others can. This is why Twice is still going nonstop. If Itzy had blown up and was selling double what Twice was then Twice would slowly be pushed to the background like Red Velvet was for Aespa.
This is why SM groups like SNSD, EXO, f(x), Red Velvet didn't need to grind for 10 years without a break. Because SM can easily replace them with a just as big if not bigger next group. Same goes for YG. BigBang, 2NE1 and now Blackpink are not full time anymore because Babymonster is doing the heavy lifting now.
Twice is still putting up decent numbers but if NMixx for example was as big as Aespa then Twice would be working part time or not renewing at all. So half of Twice legacy is based on the fact there was no predecessor to relieve them.

