My company is too small for anyone to care, but imagine that Apple or Google adopted such a plan. 5 years in, suppose it looks very likely that the Apple PSUs will pay out. I have no problem with an Apple exec borrowing money against their overall earning potential or even borrowing money specifically against the PSU payout.
The payout happens or doesn't based on the smoothed (moving average) judgment of Apple investors' reaction to those decisions and the financial and strategic performance of the company.
(Note that in many companies case, it is prohibited for employees [or a subset of employees] to enter into certain derivative contracts. Most prohibit outright speculation with naked options. Some even prohibit the selling of covered calls. I believe the full paperwork of our plan prohibits pledging these PSUs as collateral for a loan, but that doesn't prevent an underwriter from looking at my overall financial prospects and deciding to lend based on that overall picture.)
I really like the idea of "vest over 4 [-ish] years" but "payout only after a longer period of time has passed" as a way to let people earn the right to the compensation over a reasonable and market competitive period of time, but still keep them properly incentivized to create value over the long-run, not micro-optimize quarterly or annual results.