It's interesting that you mention "Lenel to Prowatch" in that list. Prowatch can certainly talk to Mercury panels, as Prowatch panels themselves are a Mercury variant. A Lenel-to-Prowatch conversion would almost certainly have left the Mercury panels in place, because ripping them out would create nothing but an avoidable expense.
Also, Brivo now talks to Mercury as well, so I would doubt that a Brivo takeover would rip/replace.
Because of the huge installed base of Lenel OnGuard, I don't doubt that SWH, Prowatch, Brivo and Hirsch have picked off more sites than Lenel has picked off in the other direction. But any integrator savvy enough in all of those products to do conversions, would also know that Mercury panels are very broadly used. If it were me, and I was converting OnGuard to SWH or Hirsch, I'd keep those Mercury panels in inventory for the next job, not the junk pile.
I have found Rip & Replace jobs from Mercury are excruciatingly rare. Why? Because changing out a system has more to do with the company's head-end software and support. The panels are just one part of it. If you don't like your system, you have nearly 20 other companies that you can go to without going to the expense of ripping out the panels.
In my long experience in this industry, I have witnessed countless migrations between Mercury-based systems, but only 1 migration from Mercury. (a CCURE customer that absolutely needed an obscure clustering feature on the iStar panel).