Had Boeing started work on a B737 replacement in the 1980s, it would have had a modern airframe comparable to the A320 which would have been able to accept upgrades as technology progressed. The A320 after 30 years is still a sound design which could easily go on for another 20 years with a few tweeks here and there. By then it will be 50 years old and the advancements made over this time period will make it worthwhile to a design a completely new type to replace it. Airbus will certainly have had their moneys worth by then.
The idea of Boeing exiting the narrow body market was previously mentioned, the B787 has been very successful for the company and whilst there are a few problems with the aircraft, it might be better to concentrate its resources in this area and address the problems within the company before looking at bringing another type into production, especially with what's going on with the B777X. The Comac C919 would be a worthwhile option for airlines in the meantime as even with the delays the program has had and inevitable issues which will be experienced with a brand new type, it's still way ahead of a B737 replacement being offered.
If Boeing started work on a new narrowbody, in ten years time it could have a state of the art design which would surpass the by then 40 year old A320. Regaining market share from the Chinese would be their main concern as the C919 would be well established and likely substantially improved by then.
Financing all of this would be a major factor as the loss of income from its cash cow together with the compensation payments due and cost of developing a new type could send the company to the wall, unless state support was provided and we know how anti the Americans are about that.
It's the old question of how long you go on throwing good money after bad.