I think it teaches you to play more efficiently, if you blow all of your shards on gear then lose your sentry, well hopefully you’ll learn from that and not do it a second time.
Aside from that, I don’t think it’s unfair. If a team has worked hard to destroy a sentry but can’t advance because of turrets holding them back, that makes their work seem for nothing as they can’t make up any ground. A losing team should fight for a comeback, not be given an advantage because they were beaten, that would be unfair IMO. As it is, Incursion typically lasts longer than Meltdown and often go the full 30 minutes, this would only make matches longer.
Back when the team that destroyed a sentry got shepherds every wave, now that was unfair.