I think you have this exactly the wrong way round. Those contract games were what Frontier was in the past. E: D and Planet coaster are deliberate attempts to change that model, and to become a 'proper' studio, not just a sub contractor. E: D and Planet Coaster (and the games that they hope will follow) are what Frontier are betting their future on.
I see. Looking into it a bit, it seems there have been several devepors over time for the Rollercoaster Tycoon series and Frontier did the third game of that series, but non of the spin-offs, like on mobile. So the new Planet Coaster is actually their own IP, looking to compete with that now.
...A more 'sensible' business plan would have been to start with the coaster game (they are world leaders at building those, even if it is not their IP), and use that as a cash cow to build Elite.
Looking at what they say about it, I would agree. and it seems to make more sense now why the progress on the core game of Elite has been slow going.
Marketing. To show that there was a market there, even if it was nostalgia driven, and to some extent to get free advertising, and also to show that demand was there to help them raise funds through other means. They didn't financially need the kickstarter to make the game happen (kickstarter around 1.5Million, game costs over 8Million, money in the bank from the IPO), but they probably did need it to encourage investment. They had, of course, already been working on it as a stealth project for some time, or we would not have been able to see the 'as it stands' game footage during the kickstarter - two devs chasing each other through an asteroid belt, and all that.
Interesting. I think if they were going to be splitting resources with Planet Coaster, as you say, likely a more polpular genre, it would have been better to extend the Elite beta at least another year. Then keep working closely with the community for feedback and focus on the core game mechanics.
If a wider released was needed, take advantage of Steam's Early Acces programme (as they have on XBox GPP) so that all new players are aware that it's a work in progress and they can help shape it further, without the criticisms of depth being as relevant.