The problem is that we tend to idealize top end content and perfect play and coordination. Which exemplifies maybe 2% of the player base. In average groups with average players who will make mistakes even with content on farm and whom just generally don't play at a pro level. A WHM will always shine because that is what their kit is made for.

Yes in an ideal environment, AST/SCH pairing is the the way to go. But for the average group just happy to complete a raid boss and is going to make mistakes and need the extra padding. WHM with either other healer role is amazing.

Don't let the composition rhetoric get you down. WHMs still raid all the time. As well as the other current non-comps. RDMs, SAMs, BLMs, MCHs, and DRKs. If your FC or Static is an average raiding group pushing for you to change classes then find a FC or Static that fits you. Unfortunately alot people have a delusion that everyone plays af the top percentile, or should be, and that's not leaving game culture anytime soon. People love cookie cutters and love to emulate the best even when it won't work for them or their peers. However there are people out there who get it and understand bring the player not the class and are happy to get raid clears even if it's not bleeding edge completion times.

Unfortunately the party finder is a sad land of wierd comp bias so don't expect to get friendly groups there often. Be friendly, open to new people and experiences, build relationships and be patient and your time will come. Don't let yourself get trapped in a social circle that doesn't work for you.