Yeah, if there isn't a fair-value young pitcher available for Happ, then of course you don't trade him, and just go with free-agent pitching.
It's not like Happ isn't a potentially valuable guy for the Cubs, particularly if he were to improve some. Zobrist is very near the end; Schwarber may get much better but may also just be a platoon .211 hitter; and Heyward's bat could make a lot of RF starts available to anybody who can hit. Also rare that both Baez and Russell are healthy and hitting well at the same time.
Tangent: Hard to find a fit, and teams rarely will trade talented pitching prospects, but could consider Happ for a minor leaguer.