Might be, apparently doing less than 60h or not being reachable during the already short holidays is not doing enough, if the memes are to be belived.
It makes more sense to split the work by topic or area or so.
But yes, follow-the-sun shifts make sense.
Many companies have specializations, and maybe only 1,2 or 3 people for each one. So at best you might have some vacation coverage.
If your pager response SLO is not that strict and is not impacted by such routines, then you probably can't justify staffing a team of 20, either.
You can do it with 5 but that leaves little room for covering illness and holidays, and you tend to end up paying overtime.
If you are getting frequent call outs out of hours, you need 24/7 coverage at a skill level high enough to reduce the total call outs to just a handful per year.
1. Multiple SREs on-call for the same week.
2. On-call chains make senior ones called more often than they hope so.