If they were selling beta access, the money-making might be outweighed by the drag of the outrage (accusations of money-grubbing, people who already paid for Horizons incensed at the mere idea of paying for it twice, claims of clear financial desperation and end-times, beta-testing being a service not a reward, on and on it would go...)
No-matter what Frontier does, they can't win
If it makes you feel any better, speaking as someone who DOES have beta access and is not going to use it, I think it's MUCH better to skip the beta-testing and wait to have a nicer more polished thing land in your lap.
If you beta test, all the thrill of discovery is offset by all your gains with it being lost when beta is switched off. Whereas if you skip beta, then your first taste of it is when it's really real and your work won't evaporate.

(Also, unbalanced and buggy gameplay sucks when you're playing a game, and betas are naturally buggy. There is a reason that people usually get paid to beta-test; it's generally an unpleasant job, much overrated IMHO.)