[JohnFen]

If it's a stack I already know I genuinely can't tolerate, I'd immediately start looking for work elsewhere.

Otherwise, I'd give it a chance. Learning new (to you, not necessarily new to the industry) things is a joy, it adds another skill to your skillset, and makes you a better and more employable dev. Even if the new-to-you thing is old and/or not popular. Actually, not being popular can make it even more valuable as there would be fewer devs familiar with the stack.

But let me admit my bias: my worst nightmare is a job where I'm just using the same skills over and over. I've quit jobs because of that. So, personally, having to learn new things is something that makes a job more desirable to me, not less.