You're exactly right. Signing bonuses - and even equity compensation - are designed to lower your TC over time. The idea is that companies offer a high TC to entice you through the door, with the understanding that at the end of your 4 years your TC will drop precipitously but at that point you'd be too invested/too afraid to interview/etc to leave.

This is why in nearly all companies equity refreshers are always significantly smaller than what would be necessary to keep your TC level. The point is to over several years converge you to a lower TC that is middle of the pack rather than the top-of-band packages FAANGs have to offer to recruit candidates.

It does seem ass backwards - and if you're an engineer who cares a lot about TC (IMO rightly so) and interview well, you will consistently cliff out every 4 years as a result.