Not exactly. In its current state and the current state of wireless, the X works great, and will continue to work great for years. The spectrum you speak of is barely utilized.
Within a year or two, there will be a significant amount of area on T-Mobile's network that will have expanded coverage due to B71. 4x4 MIMO will help everywhere, due to antenna diversity, and even more in areas that have 4x4 MIMO deployed. The X is fine this year, but a year or two down the road, it will be very long in the tooth on T-Mobile, and it's not even cutting edge on AT&T or Verizon. If I had to buy a new phone today, I'd be OK with the 8 Plus or X on AT&T, but if I was on T-Mobile, I would definitely be looking for either a cheap Android phone I could replace in a year or two like the Moto X4 Amazon Edition, or something that already has B71, i.e. the S7 Active or V30.