If the Cubs operated as a team with their market/resources "should", then there would be no reason for them to get outbid on Bellinger. He was their best offensive player on a team that needs more good offensive players.
With that said, we know the Cubs don't operate that way. They will spend money, but they will do so within some largely self- imposed parameters. So what is the opportunity cost if they were to sign Bellinger to a massive deal? What other need(s) would go unaddressed? Can they make up for his lost offense with more of a combination approach (improvements at 1B/3B) and do so with significantly less money committed for the future?
I would still prefer signing Bellinger for whatever it takes and then figure the rest out later, but all signs seem to point to the Cubs going another direction.