WotC is thinking long term. Eventually they will run out of older cards to add to historic. At that point people who only care about modern on arena could craft their deck on arena and just play it forever, crafting the occasional new standard card that makes their deck but generally never needing to worry about spending money ever again.

By making historic cost twice as much it would have helped keep people from investing in a permanent deck and instead have them focused on standard where they will always need new cards thanks to rotation.

Long term the best way to play arena is actually to just amass wildcard resources until historic has most of the cards from modern and then craft a modern deck that will stand the test of time.

I expect when that happens WotC will suddenly find itself interested in banning cards from historic/modern. For the health of the format. Nothing to do with forcing people to spend money on new historic decks.