if a significant amount of players don't have horizons, they'll probably do it.
If a large number of players don't have Horizons, doing it would be commercially risky, since it could depress the prospects for Horizons DLC sales.
If (as I suspect) the vast majority is now playing Horizons (because they upgraded or because they bought the game after last december), they might just drop the matter entirely.
I'd bet the vast majority of sales since Horizons release don't include Horizons. Steam stats show ED purchases very price sensitive. Almost all the EDbase sales have been at the knock-down £10 price. EDH looks hugely unattractive at double the price for a lot less than double the content... esp. with most of the extra content as yet unavailable and with no completion date from Frontier.