Indeed, you could argue that the tourists are kept in Phuket to protect them from Covid in the rest of Thailand!
A friend recently said that the Delta variant is a good thing. And assuming that Covid will reach everywhere and nobody (except nuclear sumbarine, space station, polar research station, ...) can escape, he is right! Delta will speed up the process of achieving herd immunity, and once that is achieved, we can finally go back to normal (whatever the new normal is).
If there were no other way to get to SEAsia, I would do it (either ASQ in Bangkok or sandbox in Phuket), but now, having settled quite nicely in Cambodia (better than expected), and the situation in Thailand worse than expected, I can wait.
There is nothing to gain for Thailand by keeping the borders closed and quarantining/sandboxing tourists; on the contrary, infection rates in Thailand are higher than among tourists (my assumption, probably could find statistical proof). When they realize this and open the border for all and sundry, I will be over the next day!
The main problem is slow and inefficient government, be it democracy or military dictatorship. I wonder how long it will take to remove all barriers to international travel introduced to combat Covid, once Covid is over.