I have 'looked" at Plametto State armory,Del-ton,Core15,Windham,DPMS and while I have ready some great warranty policy's from these smaller companies,I am unsure if they will be around in the future
The nice thing about the AR (especially the ones closer to "mil-spec" like Colt, BCM, Palmetto State, Spikes, DD, etc.) is that parts will always be easy to find and easy to replace. No big worries there. If you are going to have a problem, it is likely to happen when the rifle is new (improper assembly) - not down the road. When parts wear out down the road, they are easy to replace.
Judging by the amount of business I see every time I walk into one of PSA's brick & mortar stores (and the amout of product turnover I see on their website) I doubt they are going anywhere soon.
That said, if you are dead-set on an M&P-15 Sport, by all means, go for it. Just know that there are other options out there that don't sacrifice the features you were initially inquiring about.