I don't know why this is bothering me so much, but going into the offseason thinking that we're in this like, 9 win hole or whatever that we have to make up before we can call the team improved just strikes me as very unnecessarily pessimistic.
I don't think you can name a team out there that doesn't have somewhat significant production from last year currently unsigned/without a contract. Said another way, there's a ton of talent that was contributing across the league now sitting in the FA pool.
It seems one sided to only look at Stroman and Belli. We also 'lost' the -2.3 fWAR generated by Young, Mancini, Hosmer, Rios, and Mervis. And even if we don't replace Stroman and Belli with stars, you can still reasonably expect like, 1.5 wins from each position just from internal replacements or minor signings. PCA is projected for 1.1 in 94 games, as an example.
Basically, of course it would be nice to still have that production under contract. But the Cubs lost their 3rd best offensive player and 3rd best pitcher by overall fWAR value. Not exactly an emergency, and I'm sure that 'loss' isn't unique across baseball.