Cummins is a good example of a player who is totally committed to test cricket. Starc is another - he turned down IPL for a number of years to concentrate on tests. Meanwhile, Aus do still prioritise test cricket over the Big Bash. If you're playing tests over the home summer, you miss most of the Big Bash.
India clear their schedule for the IPL. All the Indian all-format stars play for India and in the IPL and nothing else. Look at Kohli. 374 t20s, just 22 of which have not been either for India or in IPL, most of those coming when he was a teenager. And he barely plays non-international first class or List A. I've not cherry-picked Kohli here - all the top Indians are the same, and none of them have opted out of tests, because they don't have to.
So the 'big three' don't have to opt out of playing test cricket. It's not either earn loads or play test cricket for them.
It's very different when you look at the other teams, but if West Indies, say, were scheduled for a month-long three-test tour and all the players were guaranteed a £20k appearance fee per game, that would be £60k for a month's work for most of the squad. Even in IPL times, that's good money. Times that by three for an average year of test cricket and you get £180k, with lots of time still to play other things - it's only three or four months out of the year.
It absolutely could be done, and it needs people like Steve Waugh speaking out. It won't happen if nobody calls for it to happen.