My suggestion is to open your mind to more tech stacks. Senior devs can code in many stacks, switch between them, pick up new ones, etc. I'd be just as hesitant to hire someone who refuses to touch "old" tech as I would an expert in the old tech who doesn't want to take the effort to learn anything new. Tech has variety, and senior folks should be able to handle that.

Still, this is an easy answer for you. You said it yourself - you are not motivated to work there.. So turn it down and keep looking.