They are Neverkusen

If they win the title this year, this is the peak of their modern existence. They've captured lightning in a bottle this year, but within a year of Xabi leaving they will be back to competing for a Europa spot. Dortmund and Leipzig will be stronger. Leverkusen are five times runners-up having never won it, and haven't even been finished in that position for 13 years. Their best success was finishing second for a handful of years in the late 90s and early 2000s. This isn't a historically top club.

Not sure what the league parity or Leverkusen supporter feelings have to do with this - it's a business. Xabi knows this, Leverkusen knows this and Bayern knows this. I think it's far more likely that Bayern sign Xabi than they do Wirtz (unless Gnabry and/or Coman move, then all bets are off).