"I'd leave, I even feel I'm morally in the wrong for not leaving. But the thing is that the pay is good, that I'm of an age prone to experiencing ageism in the job market, and also this is a place where I have ample slack for tuning my output and inmerse myself in side projects or personal improvement, so their loss..." - this tells me that you are a very loyal employee and you also find security more important than other aspects of your career. That is certainly your decision, but I suggest you to keep looking at the balance and consider the compromises you are making because of this. Based on my experience there is considerable talent shortage on the job market, and loyalty is seen as a positive by reasonable hiring managers.