Seniority matters a lot, but mostly in the context of the current employer.

It should not be a reason to forgo the next obvious step in your career progression, in general.

If you have specific circumstances, such as tied to one city by family, or have care responsibilities (elder care, shared child custody, spouse's job) then it might make more sense to stay. Also if you have good QOL with your current schedule and money, and no ambitions to fly to the four corners of the earth then it might make sense to stay at say a regional IF you're comfortable with the inherent long-term instability of such operators, and the fact that you be giving up more money and QOL in the future.

Skywest is about the only regional I might trust for the long-term, and even then past performance is no guarantee of future results...

But in general the majors will quickly provide better pay and QOL than other aviation sectors, and much better career stability in the long term.

It also matters how quickly seniority is moving... many junior folks have been leaving LCC and second-tier majors for the big three because the retirements and growth created tremendous opportunity to quickly move up the list a LOT. The lower tier majors don't have those kinds of retirement numbers, so it's very circumstantial. The details matter... seniority situation where you are, and the situation where you're considering going to.