I studied math and physics, not CS, but the situation is probably similar. I think the best, if not only realistic way to retain what I will call "theoretical"

I studied math and physics, not CS, but the situation is probably similar. I think the best, if not only realistic way to retain what I will call "theoretical" knowledge is to be genuinely interested in those things, possibly to the detriment of the most rapid possible career advancement. The other thing is that it's normal. As systems get more complicated, the number of interactions between N elements grows as O(N*2). Thus, most "engineers" spend the bulk of their time organizing and arranging things, fitting things together, troubleshooting, dealing with vendors, etc. In some fields, manually operating tools such as CAD becomes more efficient than understanding the algorithms programmed into the tools. This is basically human powered information processing. The work needed to make the things that are fit together, such as algorithms, falls to a small fraction of the workforce, who are not necessarily the most highly rewarded, as their work is not so visible.