Boeing made major changes to the B747 when they upgraded from the 100/200/300 series into the 400 series. Going from a 3 man to a 2 man crew with a full glass cockpit and EICAS was needed and enabled a 20 year old type to continue on for another 30 years. A major revision along these lines could/should have been done in the late 1980s to carry the B737 through to today.
Now Boeing would be looking at major upgrades to a 50 year old type with a 60 year old fuselage. Given the cost of bringing it up to modern standards and limited life remaining as it falls further behind more recent designs, would it be worth it ?
The A320 could do with a more modern flight deck, and if given one, could easily go another 20 years. The Comac C919 is a brand new design which will obviously be developed and improved over time.
Most of us have been in this situation with an old car, keep throwing money into it or bite the bullet and buy a new one.