By itself, that's not so bad. Plenty of "buy, don't build" choices out there.
However, every other would-be Mojo user also knowns that. And they don't want to build on top of an ecosystem that's not fully open.
Why don't Mathematica/MATLAB have pytorch-style DL ecosystems? Because nobody in their right mind would contribute for free to a platform owned by Wolfram Research or Mathworks.
I'm hopeful that Modular can navigate this by opening up their stack.