It really comes down to the economics of rebuild vs. new. At some point, stuff becomes so hard and costly to service it's time for something new. SLVI should also have bathrooms, that's the #1 thing wrong with the SL cars.
Single level makes a lot more sense than double level on the NEC unless double level is absolutely needed, and AFAIK, the only place they are is NJT into Penn, and thus MN/CDOT if they ever got their act together on through running.
There needs to be standard designs for various types of equipment. A good single level EMU design should be equally at home on MARC, SEPTA, NJT, MN-Penn, CDOT, MBTA, RTD, etc. Same for single-level coaches, single level DMUs, etc. NJT double deckers should stay a bespoke solution to NJT and MN-Penn. All third rail stuff is necessarily bespoke to NYC, since no one else uses third rail. The southern/western railroads already have pretty much standardized on a single gallery design and a single bi-level design, yet the NEC railroads constantly re-invent the wheel, even for stuff that should otherwise be compatible. Make an EMU/DMU/third rail design clear NYP/NJT/LIRR and work on 25hz/60hz as appropriate, then it will work anywhere.