Most devs, if HN is any indication, believe that jumping ship every 1-2 years is the best way to move up. But if you have senior devs who were juniors at the same company just a few years ago, you'll probably be more likely to stick around.
I've been with my employer for much longer than average. They're taking care of me well financially. Even the variety of projects and domains is pretty good. If I leave, it will be to declare bankruptcy on being an advisor/carrier of institutional memory.
I've had some bad experiences with this one. The devs in the senior positions had only ever worked at this one company, and they'd learnt a lot of bad habits that they then proceeded to enforce on the devs in junior positions, who despite having fewer years of experience were in many cases a lot more competent than the supposedly senior developers.
I don't think it's necessary to move every year or two, but I'm not sure it's ideal to go too far in the other direction either.
Good developers look at what they've done, and critically analyze what is good; what can be improved; and what needs to be replaced. It's these people that you want to promote and provide with enough autonomy to do what needs to be done.
It's not just developers. The old model of companies rewarding loyalty and actively investing in furthering their employees' skills has declined across the board, and it's no coincidence that the timeline correlates with the timeline of the demise of unions and collective wage/employment condition agreements.