I have had one experience on a team in HC where Sentinels really shone.
It was a 4 person team running SBB.
I was on a Sentinel, there was a second Sentinel, and there was a Scrapper and a Spider.
Basically, the other Sentinel and I were clearly paying pretty close attention to what each other was doing, because throughout the AV fights when my Opportunity debuff dropped the other Sentinel almost immediately put theirs up, and vice versa. As a result, there was basically an Opportunity debuff up at almost all times.
I'm in no way trying to suggest that therefore Sentinels are fine, just that in this one case, a small team with limited support, Opportunity seemed to pull its weight.
I still think just raising the damage is all that's really needed.