From the latest official figures from Frontier, 3M copies of the base game have been sold - and 1.3M copies of Horizons.
I doubt that Frontier would consider 57% of the potential player-base to be "not important".
How many of those who just bought the base game no longer play, or played for a while and decided it wasn't for them? How many active players only have the base game?
My guess would be that it's significantly less than 57%.
Horizons is currently £4.80. Anyone not willing to buy it at that price is probably not all that interested in the game anyway. I'd consider them unimportant.
Any major content update worthy of a full price game would probably need as few barriers as possible to do well though. If someone is willing to fork out £50ish for a game, throwing £10 worth of a game that's no longer selling at peak levels would seem like a good idea.
There will be salt. There will be endless forum threads about how unfair it is, but really, this is unimportant also.