I just took a walk along the beach, browsing proposals as I went, and I saw someone ask, "What does modular blockchain have to do with me?"

Honestly, most end users don't really want to understand DA layers, execution layers, or anything like that.
The only things they really notice are two: first, not being stuck or paying too much, making transfers and interactions smoother; second, applications not constantly falling apart just because one chain has an issue, having a bit of a "bad one doesn't affect the whole" feel.

But on the other hand, it can be a bit annoying: more chains, more bridges, a bunch of networks in your wallet, and security risks are actually more dispersed...
Especially recently, with staking and shared security being criticized as "copy-paste," I can understand.
The yields stacking up sound appealing, but if something goes wrong at any layer, ordinary people have no idea how many layers they've stepped on a landmine.

You say, "Isn't modularity useless?"
Not exactly.
Maybe its purpose is to leave complexity to the infrastructure and keep the user experience simple.
It's just that we're still in a transition period, so let's leave it at that...
And about your earlier question, "Is it just a shell swap"—I'll only say half of it:
It's not entirely that.