[outworlder]

> I've always felt that unless one has a tenuous relationship with their employer, longer tenures mean more job security.

Not really. It's sort of like a bell curve. If you have just recently joined you should be expected to be one of the first let out. The idea being that you aren't there long enough to be difficult to replace.

If you have been at a company for a long time, chances are that you have raised in ranks and received corresponding pay raises. Cutting off your position means a more sizable chunk of the budget can be freed (and replaced with less expensive individuals).

The ones in the middle are generally ok-ish. At least, the individual contributors. Middle managers tend to be replaced without a second thought. Senior leadership are more likely to survive, although sometimes they suddenly finding themselves 'pursuing other opportunities' or 'going into a sabbatical' or even 'spending more time with family'.

No-one is irreplaceable. If you are, unless you are in a handful of key positions, then the company has done something wrong.

Actual 'job security' comes from making key decision makers aware of your perceived value, so that they can remove your name from lists. Assuming that they aren't in those lists to begin with. Note that I say 'perceived value' not 'actual value'. You need to be viewed as important and more trouble to replace than it's worth, no matter how true that may be from a purely objective standpoint.