Cubs signing just a solid mid-tier SP like a Taillon-type would feel like a letdown i agree, but it would depend on how they spend the money saved between that player and a top tier SP. The difference in WAR between between them would likely be around 1.0 to 1.5. If they save 6-10m on their main FA SP and spend that to upgrade elsewhere, like the pen or catcher or a 2nd SP, and that upgrade is worth another 1.0 to 1.5 WAR, then it evens out and the point is moot. If they pocket that cash or spend it on the next Trey Mancini contract then it's a letdown.
My main hope this winter like every offseason is they take whatever payroll Ricketts gives them and they maximize the potential WAR added with that money. How it shakes out it doesn't matter a lot at the end of the day. We all like having/watching top-tier players but wins are the important thing, the rest is just emotion, and investing based on emotion is very unwise. (not saying you support that).