The real thing manufactures do is ensure that the SBC is available as long as they want it. My company started an embedded controller 6 years ago, at that time

The real thing manufactures do is ensure that the SBC is available as long as they want it. My company started an embedded controller 6 years ago, at that time we got a contract from manufactures that all the parts would be available for 12 years, thus ensuring we won't have to redo that controller for 3-4 more years (and we might just renew the contracts). As a hobbyist you don't have that guarantee (except the 3 you named) it happens all too often you buy something only to discover not enough other hobbyists have the same board to give it a critical mass of people to make it work. It can still work, but you will personally have to do all the work to make it keep running which few hobbyists have the time/energy for (and often the deep technical knowledge is lacking as well)