AFAIK, there aren't any plans in Azure to give up ADO in favor of GitHub. If anything, with the push to standardize builds internally, it wouldn't make sense to move to GitHub for at least another 2-5 years.
Obviously, I don't speak for my employer and leadership may have other directions in mind.
Azure DevOps and Github largely cover different, though overlapping market segments.
I would be slightly more concerned about Github Enterprise and Devops co-mingling over time, as I think that may be inevitable, which makes me concerned over the public/free resources that Github offers in the long run... even then, migrating to Gitlab is an option should that time come. My only hope would be better discoverability and social coding with Gitlab to better match Github over the interim time.
Even then, it's just a possibility and somewhat unlikely that MS would burn this much karma.