If you're serious and really don't want to leave (though to be honest, tenure probably isn't as big a deal as you think - easily explainable too since you can j

If you're serious and really don't want to leave (though to be honest, tenure probably isn't as big a deal as you think - easily explainable too since you can just say you literally weren't being compensated for additional responsibilities) then maybe ask for a demotion? You could even list both titles on your CV for one length of time at the company, and just let people assume you progressed to senior rather than 'regressed'. But ideally/presumably that doesn't happen - use it as negotiating point I mean - you can say you're honestly willing to be demoted, because it's going to spare you work, to the detriment of the company, with no loss in pay. Hard to imagine they wouldn't see sense and pay you more, and if not, try to shrug and enjoy the 'easy ticket'? (Especially since you phrase it as them being overpaid, not you under, so you'd presumably be happy with that.)