It depends. 1–2GB graphics files probably won't be much different.
If you were doing music or video production, large graphics files, Photoshop etc., then I would recommend trading in.
Nonsense. There's nothing in the new machine that will cause it to have a longer life cycle than the old one. No T2 chip or other components that distinguish it from its older siblings. Newer CPU, that's all. It will be a few days newer otherwise.
I'm not saying don't trade it in. Yea... we all want the latest and greatest but I see no technically compelling reason for the OP to do so.
None.