IAAPHO (I am a pediatric hematologist/oncologist), so take my comments for what they're worth...
TL;DR: This is a big freaking deal.
To address your points, ALL was the first drug that CAR-T treatment was approved for for several reasons. 1) It's common, so easier to do the trials. 2) It has good data to support the use of immunotherapy (see blinatumumab, inotuzumab ozogamycin), and this therapy is really just the next step in a long line of improvements in immunotherapy. 3) It's got a good target. Almost all Pre-B ALLs express CD19 and downregulation (the main resistance mechanism for immunotherapy) seems rare (but has been documented in relapsed cases). 4) The cure rate is nowhere near as high as you cite for older children and young adults (currently less than 80% for anyone over 13 years.). THIS is actually where most of the use for CAR-T therapy will come from.
The bigger news is that the TECHNOLOGY for this treatment has been FDA approved. Once you have perfected the cell harvesting/transfecting/culturing/infusing process, it's trivial to plug in a different antigen target into the cassette. And in fact, this is already happening on a large scale. Hop on over to clinicaltrials.gov and search for CAR... We've already got some results from GD2 targeting (neuroblastoma), HER2 targeting (breast cancer and osteosarcoma) and IL-13R2 (glioblastoma multiforme) with promising results. And remember, this is just the first generation of CAR-T therapy to make it to the market. As the technology matures the acceptable uses of it will broaden.
Your zero-sum game argument has been sufficiently debunked below. But suffice to say, this is a true breakthrough technology which will have a huge impact on the field for years to come. But you've gotta start somewhere.
(In bigger news, I think this is the first time in my 18+ years on /. that there's been a pediatric cancer article. That should tell you something...)