They will keep adding titles via updates if necessary.
MS is so ahead of the curve they have their BC already implemented in the system from all their work this gen and i am assuming their beast hardware is also bruteforcing games to run. So that helps too.
Sony's ps4 BC is different because they are basically starting from scratch. It seems fw updates will be the way to add more titles but it does look like they built the system (with AMDs help) to get a native BC mode running. Most will run fine. Games with a "ps5 boost" will need updates. New ps4 titles will either run natively or need a patch to work.