Segregating players away from such a large portion of the game, such as planetary landings would be a stupid idea. This was a core feature within the second game and is being offered for free by other games and engines - space engine have designed their game for it example and did it all for free.
As it is, Elite as a game is nothing more than a very polished beta - as such, people who have paid full retail price SHOULD get these expansions for free. This keeps existing players playing the game - it also gives them a feeling they are valued as paying customers, and it doesnt involve any more pay walls to get newcomers into playing the game.
Frontier can make enough money off selling skins, custom variants of ships that are limited runs, and in these expansions there are enough avenues for making even more money off customisation. Want to bling out your ships interior? can do a basic job with creds, but want some super swanky custom interior? money dollar shop.
same on the ground - if they ever allow building colonies, they could sel lpremium interiors, land vechiles etc etc.
People WILL pay for these. This is how planetside, mwo, etc etc work and make more than enough money. If Frontier are needing money for an expansion, im sure if they were to also ask for donations on their site to help with development, people would gladly throw more money at them if they market it right now the concept and engine have been proved to be excellent. I dont see why more companies dont do that, like os projects, and have a link where you can continue to support them for the sheer sake of it. Id gladly throw £5 a month extra their way to help with development. That would add up if a hardcore number would also do the same and it was an available option.
Just my two cents. I find this a much more innovative and better way forward for both players and developers.