Here's the argument for Alonso, in my view:
The Cubs have above-replacement efficiency in a lot of places, but the thing they need more than additional defensive value(especially without Bellinger) is a star level bat. Alonso could provide that without costing as much money as Soto, Bellinger, or Ohtani, and have a lower trade cost than Soto too.
As with Bellinger, Alonso is an easy QO decision so any trade cost you can partially make back through the QO, which is noteworthy as the big contract rolling off next year's books(Stroman) can't be QO'd. Or you extend him and have that player for longer than their trade cost implied.
Alonso's WAR is held down significantly by his defense, which OAA hates. As with catchers, there's an argument to be made that 1B defense isn't perfectly calibrated by metrics, and his career wRC+ with averageish defense is close to a 4 win player
Alonso's home park is quietly one of the most pitcher friendly in the game, and he could see an uptick from not playing so many games at Citi Field. Similarly, he's only ever played for one organization, and one that is not heralded for player development, so fresh coaching perspective could conceivably lead to improvement(note: this can go the other way, e.g. Taillon's first half)