Phenylephrine was the replacement that doesn't really work but is non (or less) stimulating right ?

From what I remember it was actually quite effective topically but not through pill form. Could be wrong.

Also makes me wonder if there's an alternative function to DXM for people with colds (maybe it makes them feel better in other ways). Or it's just good marketing and associated with NyQuil having other drugs and people assuming DayQuil works

It's not that it's less stimulating, it's that pseudoephedrine basically is methamphetamine (the chemistry to reduce it to meth is truck-stop straightforward). But oral phenylephrine doesn't work at all.

The case against DXM is nowhere nearly as good as the case against phenylephrine; phenylephrine is a scam, and DXM is a drug everyone thought was the gold standard cough suppressant, but then serious studies knocked down its effectiveness.