It is interesting to compare him to other free agent catchers and their deals. If you look at it that way he probably is getting too much money. But he seems to be the best Catcher in baseball now, right? Those other guys, other than Posey and Mauer who got BANK couldn't say the same. The position must be down compared to recent years but 5/115 doesn't seem so bad in that context.
Also, compare him to Springer at 6/150. Is Realmuto at 5/115 really worse? They seem like similar talents to me. I think the Phillies probably did fine but there's no doubt that they are paying the FA premium for him and that will always carry a good bit of risk.
You raise a good point about his baserunning though. That's the one issue that could really make them regret this. You would think that his foot speed would crater as he ages. 30-somethings always do, but do catchers get slower faster? Like you said he's only a good defender and a good hitter, if his baserunning craters his on field performance could take a significant hit.