Wait. When you say Pay-to-Play, do you mean a one-time fee like most games or a subscription model like WoW has?

If it's a one-time fee, and microtransactions were to be completely removed (basically, goodbye Supply Depot, Energy Market and Energy entirely), you'd be looking at an incredibly long grind that would be very repetitive with no way to skip the timesink if you don't want to grind or simply don't have time.

If it's a subscription model, everyone would just leave because no one wants to hang around in a game with very limited endgame content if they have to pay $15 a month to do it.

Either way, without seriously major rebalances that would probably end up being better incorporated into an entirely different game (Spiral Knights 2?), SK would fall flat on its face as a P2P title. Almost every major system in the game is designed specifically to work with an F2P model, and removing the F2P aspect makes people take a different look at the game entirely. If they had to drop $20 to play SK in the first place, maybe being able to play the Rescue Camp or something as a demo and only looks at the rest of the game through trailers and reviews, it only looks mildly appealing. If they actually did decide to play, they would find an imbalanced weapon system and either extremely repetitive content (arcade), extremely linear AND repetitive content with a cheesy story and no voice-acting (missions), or imbalanced, newbie-unfriendly PvP(Lockdown). And if they made it through all of that along with the extremely tedious gear-gated tier progression, they'd finally find themselves at and endgame that has been almost completely stagnant for close to three years and a development team that is too underfunded and understaffed to develop endgame content at and acceptable pace for an MMO.

Bottom line, SK doesn't have the content to justify paying to play. It is a Free-to-Play MMO that was developed for a Free-to-Play model, and F2P is too deep in its core (hahaha pun *dodges tomatoes*) to rebrand it as Pay-to-Play. On top of that, no game ever goes F2P -> P2P. P2P games only switch to F2P when they've stopped making money. :P

After july 30 2013 patch game became f2p, but you still need energy for slots and keys.

Trinket and weapon slots can be obtained via event boxes (ex: winterfest). SL keys can be obtained from forge prize boxes (super rare). With the exception of featured auctions, the only items that cannot be obtained without using energy are promotional items, silver keys, heat boosters, and extra evo catalysts (that I know of).

Even before that patch, you could find higher star items in the clockworks. It was still technically possible to achieve vanguard rank without using energy[ or tokens for boss rewards].
That patch did make it significantly easier for F2Pers to play the game.

"So a P2P game would have endless content and constant new stuff?"

a developed-as-P2P-subscription-model-from-scratch game would have stuff at a more frequent pace than SK does. SK as a P2P game would have no new content whatsoever, because it would have about 15 players.