Not acquiring another high-leverage arm doomed the Phillies bullpen
The Phillies bullpen was one of the best units in baseball in the regular season, but the postseason is a whole other beast. In reality, there weren't many arms in there that Rob Thomson could completely trust.
José Alvarado was their best reliever by far in the regular season and postseason, although the run he allowed in Game 7 was huge. Jeff Hoffman was used often and was great outside of the Austin Riley blast in Atlanta. Who was next in the pecking order in terms of relievers you could really trust? Matt Strahm?
Craig Kimbrel's playoff history wasn't good entering the season, and his Arizona series was an absolute nightmare. Seranthony Domínguez had a good postseason, but his inability to pitch in high-leverage spots in the regular season forced Thomson to avoid using him in big moments. This is why the Phillies had to turn to a guy like Orion Kerkering to pitch in the seventh inning of a one-run game in the NLCS. That went about as well as anyone could've expected.
The Phillies needed another high-leverage arm to pair with Alvarado and Hoffman. You can somewhat understand trusting Kimbrel, but even then, they'd still need another arm. There weren't enough late-game arms for Rob Thomson to go to. There's a good chance they would have won that Arizona series had they signed or acquired just one more reliever.