The problem with giving out money is that no one purchase blows your budget, no one second tier signing gets you to the Lux tax, but you can quickly eliminate your flexibility by giving out too many overpriced/overlong deals to second tier players.
Happ is not a big upgrade over what we have in starting pitching other than being a LHP, the value is you can then trade one of your SPs for another asset, the downside is if he gets $20M a year you may regret it in 2021 if you're getting close to the Lux Cap and it makes it harder to make moves just as you become really competitive.
If Klenak expects to make multiple signings/trades the next three years, he has to be wary about buying everything on the menu, especially if he has to overpay.
McCutchen was an overpay, but moving Santana defrayed much of the cost ($20M to $17M, but Segura added $14M, so still net $11M more). However, his value was that he is replacing a "hole", the other OFs right now struggle to get to .700 OPS, he's a good bet for a year or two of .800 OPS. So the marginal value was greater for the Phillies than most teams.
Happ will replace a young pitcher with more upside who posted a similar FIP, so not a lot of incremental value, thus less reason to overpay.