Fortunately, as bad as this is, I don't believe many companies will implement the worst version of it. (Server side + subdomain + different name scripts)
The reason is that we had server-side analytics available for years and virtually every big website still implements the clientside part. If they can't be bothered with that, I don't expect they'll move the whole tag manager any time soon.
I have to agree. Working as consultant/data analyst none of the clients I know (most of them on the paid 360 version) are anywhere near to switching.
Complexity as well as the price tag for the proxy is keeping (even is it would be just a fraction of the 360 bill) keep them from jumping. But mostly the complexity and effort for the migration.
If the were to start from scratch they would probably go for it.
Additionally most data privacy departments actually have some influence nowadays. They would not stand by if marketing were to implement this and not honoring consent.
The reason is that we had server-side analytics available for years and virtually every big website still implements the clientside part. If they can't be bothered with that, I don't expect they'll move the whole tag manager any time soon.