The only way I'd be for a combination would be if it did not use every player from both leagues, but a select group. Take a portion of the 80s players, and a portion of the 70s players, and make a combined group of players about as large as the 70s player pool, give 'em all 5 possible seasons and go with it.
That would not be a replacement for the 70s and 80s games, just another option.
On that note however, I, and I assume most everyone else would much rather see a 90s, 50's/60's or even an ATG mystery card game over a combined 70s and 80s game.