The problem for airlines is that by and large for your average leisure passenger the environmental impact of climbing aboard an aircraft for their weekend break in Barcelona, or two weeks on the Turkish Riviera is so far down their list of priorities as to be virtually non-existent. I avoid flying when travelling to the near continent, not because I have bought a membership the the Green Party, or have mates who block roads in the name of "saving the planet" but because the hassle factor of putting the (polluting) car on to the (allegedly green) Le Shuttle train is lower than flying. That being the case airlines aren't going to attract passengers on the back of having acquired the latest aircraft that have a lower environmental impact, and certainly aren't going to pay more for their tickets to travel on a shiny new 737-MAX than a written down 737 Classic. The only incentives to airlines come from the bean counters that can see an investment in modern equipment will reduce their fuel bills and therefore their seat-mile costs, and at some airports benefits in lower charges for operating modern aircraft. Aside of that the
PR departments can use modern "green(er)" equipment as a method of upping their airline's profile.
Perhaps the best way to push a move towards greener flying is by international cooperation on taxation for older generation aircraft, or conversely incentives for newer aircraft but this is hardly likely to happen with so many, not the least commercial, vested interests. I don't see taxing passengers extra to fly is really a starter since, at least in Europe the taxes and charges on tickets already often exceed the raw cost of a ticket and the passenger simply won't wear that when it comes to the ballot box.
Although it is really tinkering around at the edges some taxation on leisure flying would send a message, and in my view business aircraft should also have their climate impact recognised by levies that really ought to drive some of the users towards the front end of commercial flights rather than flitting around the globe in glorious isolation, leaving an excessive carbon footprint in their wake.