I think there is a an interesting request here.
Officially the answer if you have existing work out there that was done in 2.79 stay on 2.79. If you want the new work you are going to have to migrate.
It is easy (ish) to rip out the renderer, it is another task to wrap it in a manner that it can be used again in a decoupled way. The word from Ton Roosendaal, who wrote it in the first place, is that the code was 25 years old
This feels very similar to the python2 -> python3 move. It was assumed that people would migrate to python3 over time. It original EOL for python2 was 2015, that had to be bumped 5 years because there was so much legacy work deployed that it was not considered feasible to sunset python2.
At the moment blender 2.80 has not been officially released, so technically the clock on the 2.79 -> 2.80 has not even started yet. However with all the news in the past year about how blender is finding it way into commercial tv and movie production it may begin to rattle out how hard / expensive or unready it is for people to migrate.
So the question for the OP and anybody who gets paid for work that they use blender for. If you want Blender Internal, how much is it worth to you or how much will it cost you? If you are either stuck on 2.79 and can’t take advantage of new features in 2.80 or you must take the time out to migrate to 2.80, it has an associated cost. Would you take that money and put it towards and effort to port Blender Internal? If so, how much? a $10, a $100 a $1000?
As an FYI this is the definition for a business case:
a justification for a proposed project or undertaking on the basis of its expected commercial benefit
Well? What is the commercial benefit?