It's described more in the report. Their assessment #6 hinges on

- "Very likely" poor or non-existent robust GPU support,

- Unlikely robust support for specialized processors (i.e., non-general purpose CPU) like "Data Flow processors" and "processing in/near memory",

- Fortran advocacy and advancement is likely limited to DOE, which the report implies is insufficient, and

- Vendors of hardware have prioritized C++.

I can see this.

About 4: I think Fortran looks quite like C when it comes to the CPU architecture. Both are somewhat low level procedural languages.

As long as C works well on future CPUs, Fortran is probably fine and I don't see C disappear overnight. At least, Fortran compilers can be updated to produce reasonable code for new CPUs. Which should happen because of the shared compiler architecture (backend).

About 5: GPUs architectures seem less stable and seem to require specific development. If those specific developments require language support, that's probably not coming to Fortran if the assumption is that Fortran is going maintenance-only.

About 6: advances in computing technology are not restricted to CPUs and GPUs.

(disclaimer: I've only seen Fortran code from far away. And code targeted to GPUs too).

I guess “advances in technology” are not only new CPUs and GPUs, but also many other kinds of hardware and software

"computing advances" probably means new software and algorithmic techniques, not (only) new hardware.