The OP (me) did not say that LTE is obsolete once 5G comes out. He (me) said that 4G phones would be obsolete...as in “out-of-date.” Which they will be. The 5G phones can access 5G and LTE networks. 4G phones will not.
That isn't really obsolete then.....
When AT&T shutdown the original iPhone's 2G network two years ago, that would probably be considered obsolete. But, if LTE iPhones will still be used and sold after the 5G iPhone drops, I personally wouldn't the LTE ones obsolete, and I doubt most people would....
But getting to the original question on why by the 2019 LTE iPhone when there might be a 5G iPhone sometime in the next few years, there were plenty of posts that made valid arguments on why buy an LTE iPhone in 2019 imo....
I would say lets wait and see what the 2019 iPhone before making buying decision...
There will probably be lots of reasons not to buy a new iPhone in 2019, but I suspect that Apple making a 5G iPhone sometime in the next few years will not be an important enough reason not to buy now.