My personal opinion is that as long as STs don't give up big plays then that is adequate - and that is largely on coaching and discipline (Stay In Your Lanes). I think ST coverage teams are more and more minimized in today's game - factor in kicking kickoffs out of the EZ and punts with no returns (directional kicks, fair catches, or 82 yrd kicks for touchbacks lol) and you are talking about maybe a handful of plays (not even counting having an elite offense that literally doesn't even punt some games). As long as coverage teams play disciplined as a unit and don't give up big plays, and returners are well-coached and don't make bone-headed mistakes - then I think you are covered. I think it's an age-old response to say how imporatant STs are - they are important, but I think you can have adequate units without keeping a bunch of ST-only players - which is also fine if there's no one else that would make the roster deeper/better at regular positions. But on a deep talented team like ours, I would much rather keep a skill player who might contribute if there are injuries etc over the course of the season.
My real pet peeve related to this is why rosters aren't say 55 - then you can potentially keep a couple ST-only players and deep, well-constructed teams aren't penalized