They might need to think about getting a return flight ticket to their countries so that they can prove they need to stay here longer till the date of their departure so that it sounds more credible. If their flights got cancelled, and they needed to reapply for more future COVID-19 extensions, they should show a cancellation email to the Immigration Officer to prove it. As for those who arrived here in March 2021, according to my estimation, we might start having problems to get such extensions granted around March-April 2022.
I arrived in Thailand, but not in March 2020, but March 2021 and I have been on COVID-19 extensions since then. I plan on leaving the country around June or July if nothing happens.
Please, keep us posted. I am conducting a qualitative research based on the info gathered from different people on COVID-19 extensions to ellaborate a map of likelihood percentages by nationality.