I don't think it's very complicated, really (and I also don't think Kapler has had any impact on personnel moves). Klentak was very transparently trying to follow the Cubs/Astros rebuild model and timeline, including the judicious addition of veterans one year before the team was truly ready (Santana/Arrieta). Problem is, he never had an Altuve or a Rizzo/Bryant, and nobody developed into a Correa or Springer either.
Had last season been one in which a healthy J.P. Crawford locked down SS (even with a lowish OPS), Odubel Herrera won a gold glove and a batting title, Maikel Franco put up 2015 numbers for an entire season and Altherr/Williams both thrived we might have had ourselves a nice homegrown team, augmented by one mega free agent and McCutchen (who is really just replacing Santana). Instead, as a group, they (Alfaro included) look like a collection of 2-3 year question marks (and Kingery is one as well).
Meanwhile, the landscape in baseball shifted. Opportunities presented themselves. Segura was cheap (and took Santana off their hands to boot). Realmuto was unexpectedly available (if not cheap).
And the landscape in the division has also shifted. Finishing behind the Mets this season was not going to be acceptable. Hitting another bump and maybe not being better than the Nats and Braves - both of which have superior young talent - in 2020 as well as 2019 was not going to be acceptable. With so many teams sitting out free agency or even contention the iron was hot.