I think this map pool just isn't really sentinels favored at the moment, leading to a very deflated sentinel pick rate. Icebox and Ascent were two maps where sentinels were almost must picks (KJ had a 91% pickrate on Icebox in Toronto, the last major event when it was in the map pool, and vyze had a 68% and cypher had a 32% pickrate on Ascent in Champs 2025), and now they're out of the map pool. Compare that to nowadays where Bind and Split are two of the most popular maps and they always have been maps where double controller no sentinel has always been popular. Plus I think that we are going to see a rise in chamber usage bc honestly I don't think everyone has realized yet that one of the best paths to winning is abusing the outlaw and OP right now and chamber is one of the best agents to enable that.
Maps like Haven and Pearl still do show that sentinels are fairly popular, they just aren't as universal as duelists, initiatiors, and controllers are. Locking down sites is a job that controllers can do on certain maps, but on maps where your defenses are stretched very thin like Haven or Lotus where you have to defend 3 sites or maps with massive and complex sites like Icebox having a defensive specialist who can passively control more ground is very important.
TLDR:
I don't think sentinels are doomed, they just have a shit map pool and are inherently situational