The reason given for de-orbiting it is that it does not have the resources left to support any more missions. In that, I take it to mean things like oxygen, water and other consumables.
The TG1 was never designed for a long life, and as such, I think re supplying it would be too much of a hassle and China decided it would be better off to just start with a clean sheet design that was designed and built for a long life from the start rather than waste time and resources flogging a dead horse.
The automatic docking tests have pretty much cleared the way for unmanned supply missions for a future space station, but without a permanent crew, you would have to leave the supply module docked with the station so the crew from the next mission can unload the supplies, but that requires at least two docking hatches, which the TG1 does not have afaik.
There is the possibility of leaving the TG1 up there and than joining it to the new space station as a cheap land expansion. I'm sure the taikunyuans will welcome the extra room, but I'm not sure how much the TG1 would add to the new station as a whole. You also have to keep in mind that the TG1 only has 1 hatch/airlock, so if you attach it to the new space station, it will become a dead end where you can add no more other modules onto it. Depending on the scope of the Chinese plans, they may want the flexibility and growth potential of being able to add additional modules.