I myself see no reason to delete it,since it will take time to port over tsw2020 content for the senario planner,plus tsw2020 doesn't suddenly become any less enjoyable to play just because tsw2 has come out.
Plan to keep as it will take time to get content moved over, and even after that probably keep it as NEC will be the only content not being moved over.
I’ll be keeping it as well as downloading all the preserved content into TSW2. The only reason I can see to delete it would be for storage space but it’s less than 50GB on PS4 and I’ve got 2TB on an external disk to play with. With the distinct possibility of routes picking up extra bugs in the transition, I don’t want to delete the versions that won’t. I don’t know how much I’ll actually play TSW2020 once all the preserved content is out but there might be a difference in how they play so I want the option there.