TL;DR:So finally let's talk about what will happen if the implemented system
of deckslots is inadequate to my needs... and the answer is obvious: I
will barely use it.
Jaya and Teferi have had the level 60 rune slot at 2000 and it's been _great_. I deliberately levelled them to 60 just to get it. It's finally a reasonable cost and should be extended to EVERY planeswalker. All new ones but also all old ones retroactively.