I'll disagree with that. The solo monster concept took some time to figure out, but I think its chassis is superior to legendary monsters. Later 4e solos are pretty cool, with all sorts of neat abilities to trip up a group of players. Meanwhile I feel like 5e legendary monsters haven't gone anywhere, its the same fundamental thing, get a few actions outside your turn, get legendary resistance which I think is a pretty poor mechanic because its very biased against spellcasters (especially if there is a single spellcaster in the party).
You are saying two different things: superior chassis and then describing cool powers. Cool powers are chassis independent. I think the LA chassis is better than the standard MM3 solos. I do agree with you that LR is not always the best, I like how LevelUp did it - on the Legendary chassis.
I agree WotC hasn't been the most inventive with Legendary monsters (though just like 4e they have gotten better), but the chassis is solid and others are doing great things with it. The LevelUp monsters, and some of my own, can really make the chassis shine. Just adding the mythic* version was a great addition, but there is nothing stopping anyone from adding "neat abilities" to any legendary monster. There are a lot of legendary monsters with cool abilities and some of them are even by WotC.
I will remind you that prior to MM3 solos had a serious action economy problem. The "instinctive action" was a pretty neat solution, but, IME, three off-turn actions is better than 1.
Now, the real issue with 4e solos, particularly at higher levels and that still exists in 5e, is that they just don't hit hard enough. This coupled with huge amount of HP still made them feel like bags of hit points for some people. 5e monsters have relatively less HP, but they can also, IMO, do more cool things because of their legendary actions. YMMV.
*There is also the multi-stage monster Auril which is an interesting extension of the legendary/ mythic concept, which has 3 stages each with its own statblock. Hopefully it is not a one-off like the 4e Lolth was.