Most numbers are malleable, so you can probably justifiably predict whatever you want for sales.
The most important determinant of how well the game sells will probably be how broad the appeal of the game is when taken at "face value".
Players that already know/like D:OS, D&D, or the original BG games are an easier target, so long as BG3 goes at least some way towards meeting those players' needs.
But, there is a much larger potential player base that likes fantasy games, but prefer fast-paced action rather than the more deliberate-pace of typical cRPGs ( For example, Neverwinter MMO is aimed squarely at a portion of that wider market and has been quite successful ).
I think the game, when it finally appears, will be adored by some players and vilified by others, but probably review well in the mainstream press unless Larian get completely tied in knots trying to appeal to everyone. Sales in the 5-10 Million range wouldn't surprise me, but beyond 10 Million would.
Obviously, the sales into the non-D&D/cRPG audience provide a potential for expanding the DnD/cRPG fan-base, but I wouldn't expect a huge cross-over, as most other cRPG games simply don't achieve the BG3 production values.
