It's a win win either way. I'll bet you 80% plus would switch but the remaining still have an option. Sticking to the middle means bleeding from both directions with no forward progress. The terrain is markedly dated in appearance now and will become more so each passing year.
For argument's sake, we'll use your numbers.
Not being sarcastic, but looking at that from the other side, it's a win if you effectively guarantee 20% of the playerbase gets alienated?
If I logged in at prime time and the MA had 80 people in it while the HD one had 320, I probably would go do something else. Most if not all of that 20% probably would as well.
Improving gameplay and upgrading terrain would do a heck of a lot to retain current players and bring many in the door which serves everyone's best interest.
But would it cover that 20%? This game is niche gameplay. Many who try it don't stick around for a myriad of reasons. Lack of graphics is probably pretty low on the list.
A lot of people say people are going to leave because the graphics look dated, but do you know of anyone who has actually cancelled their account and said, 'I love the community, I can't get enough of the gameplay, but my god the lack of new maps and dated graphics are just something I can't get past.' I haven't, most people I've seen leave were sick of sandbox gameplay and the behavior it fosters in people.
Wiley.