What you're forgetting...
Its great you have a nice package with TM-US.
What you're forgetting however is that it is a millstone around DT's kneck and that AT&T deal might possibly have been the best deal for you. TM-US don't have a nice package with you.
TM-US isn't performing well enough for DT to want to keep it . They're limited in what they can do in the way of LTE expansion (going as far as to call HSPA 4G to try and seem competative).
At least with AT&T there'd be no change in handset or SIM required.
One way or another TM-US is likely to go away. Its been more or less on the market for some time, and this seems to have been the only nibble.
Its a short term victory that leads in a long term defeat for the customer.