Keeping so many designated roster spots for ST-only positions has always been a dubious proposition and has hurt the Patriots in the past, made especially egregious with the new KO rules last year. Keeping Ebner, seems like a keen waste of roster space.
I'm convinced special teams is like baseball defense was in the Moneyball era: people didn't understand it and there weren't readily-available statistics, so they assumed it didn't matter, and you had people arguing things like, "would Frank Thomas' bat at SS offset his glove?" Fast-forward a few years later, and people start understanding that no, it wasn't unimportant, we just didn't get it.
The Patriots averaged 70 offensive, 65 defensive, and 28 special teams in the regular season in 2018. So ST makes up ~17% of plays in a typical game. That's nothing to sneeze it on its face, but beyond just play count, special teams sees more yardage change hands than a regular play, and bigger disparities between the top and the bottom unit. I would argue a typical special teams play is less important than a third down or red zone offensive or defensive play, but more important than a first-or-second-down O/D play elsewhere on the field.
I think we also forget games that turn on a ST play. The Patriots won the Chicago game last year by 7 points. They had two ST TDs. Two blocked punts kept them in the Miami game despite appalling defense. We've also seen, more rarely, ST disasters lose them games, such as 2015 against the Eagles.
At this point most NFL teams have a similar number of predominantly ST players. The Pats will pay those guys a little more money (e.g. Slater) and fill the role with players not on rookie contracts, but it's not like the Bills or whoever don't have three-five guys who have no role other than teams.
Every team ultimately has to account for a similar number of snaps across their units with a similar number of players. I think it's a question of prioritization. Usually your deeper bench guys are going to play way more ST than O/D. Some teams are fine with having developmental O/D guys who may not be effective ST players. The Patriots reason, logically enough, that if that guy's going to make most of his contributions on STs, he should be a player who is good at STs. There are occasional downsides to this approach, but I'd argue it's been more good than bad by a substantial margin.