While GitHub had previously started work on migrating parts of its service to Azure, our understanding is that these migrations have been halting and sometimes failed And there's no reason to suspect this next batch of migrations will be any different. Telling your engineers, 'good luck, you get to spend the next 18 months treading water,' is a terrible way to get them to give their best or even stick around.
I think sometimes the migrations were halted more because MSFT wanted to hold off. Microsoft makes more money selling Azure outside the company, and they needed more power for GPU build-out once LLMs and AI started becoming one of Microsoft's Things™.
That said, the difficulty of the work was absolutely also a factor in deciding not to carry through with earlier migrations, so your point still stands as a whole IMO. Just, now solutions will be found for blockers and engineers will be kept on it, rather than efforts stalling out and being put on hold.