i think GTA6 (if its good as 5 or better) will sell in the 35-40 million range. i doubt it will beat the 52 million already set by GTA5. it had the benefit of the next gen remasters with the added FPS mode.

i think GTA5 will always be the best selling 60$ game ever.

Unlikely. GTA V has the benefit of being on both current and last gen; with a staggered PC release. I'd be amazed if GTA VI had such luxuries or if it was to sell nearly as well (though it would still be a major success, ofcourse).

I suspect after V they will purposefully start releasing the games at the end of each gen and do a year after port job to the new systems. In reality this game has likely sold to 35 million people or so, but a large chunk of those people double dipped when the new consoles appeared. It did help that at the time both systems were a barren wasteland.

i don't see this happen unless they relase it at the end of this gen and do a remaster vor next gen + pc port later again

xb1 + ps4 userbase should also be lower than 360 + ps3
but the game will still sell gangbusters and over 30 Million ofc maybe up to 50

GTA 6 will sell less than GTA 5 because the X1 + PS4 user base will not be that large. GTA 7 could possibly sell the more than V though.