There is an understanding of how it impacts your time, your energy and your life that is impressive? To be honest, I feel bad for being so macho about oncall at the org I ran and just having the leads take it all upon ourselves.
Personally I think this is a fair system, and I would hardly call it slavery.
(disclaimer: am Google SRE)
25% time for carrying the pager was to compensate for: 1) Requirement to be able to get to the plant in 30 minutes. Fresh snow? Too bad, no skiing for you this weekend. 2) You must be sober and work-ready when the pager goes off. At a party? Great, but I hope you like cranberry juice.
As the customer who signed the time cards for the pager duty, I thought that was not only fair, but it also drove home to me as a manager that the cost was real and was coming out of my budget, not some general IT budget that someone else took the hit for. This is one case where "You want coverage for your service? Give me a charge code for the overtime." was not just senseless bureaucratic friction, it led to healthier, business-driven, decisions.
On a slightly different note, "low-level team to have at least one engineer on call at any given time" - this line itself is so true and at the same time it has so many things wrong. Not sure what the best way to put the modern day slavery into words given that I have yet not seen any large org giving day off's for the low-level team engineer just cause they were on call.