I don't see it being a permanent feature ever, not even with DM. I think they would implement an item which gives this feature as a temporary buff for a duration for an X amount of days, depends on the tier of the item you're buying which gets more pricy as the tier gets higher. All of that assuming they would even implement an item of this nature because it seems like they want to keep the training room not over powered.
But this kind pf feature has its flaws as it would have to be a DM item which means mostly P2P players would get them and get stronger faster, making a bigger gap between P2P and F2P.
I'm pretty much mumbling by now. I don't even know why I went so in-depth with this idea I'm not even an P2P player
Edit: Going even further with this idea () - that flaw I talked about can be solved if the item wouldn't be available in DM but in other methods like rewards for events or pvp, rare drops in certain events like the SP tickets being a rare drop in the race events, maybe even implementing them to FP and SP tickets eggs (I'm sure no one would mind as they would be items so they won't take any digimon storage space and as they can be over powered)
OK now I'm really done. Thanks for the super long read!