Stinger:
SH-60R and CH-60S, later renamed MH-60R and MH-60S, were two different ideas. I won't talk to UH-1Y and Z, but I seem to recall that they were initially envisioned in a similar fashion, as a rework/upgrade versus new product. Why was this? MONEY! (And, being Naval Aviation's second class citizens, aka helicopter sorts).
a. CH-60S was part of the Helo Master Plan of the early 1990's, which replaced (after much gnashing of teeth) CH-46 Vertrep capability with a Black Hawk variant. Why the Black Hawk variant? Because using a Sea Hawk variant (and the issues with forward tail wheel and cargo requirements) would run about a million dollars more expensive each, and this crap went on during the Clinton Administration when there was no forking money unless you were Super Hornet or E-2 ... OK, I am not bitter, I really am over it.
b. SH-60 B/F to R was originally envisioned as an overhaul/depot level remanufacture program because ... wait for it ... there was no forking APN1 money, but there was repair money, during the Clinton Administration. As it turns out, a few old hands at the OEM suggested that a new buy would be cheaper and better use of scarce dollars. In the long run that turned out to be true, and became the ugly truth when the first few went to Troy and the Lot 13 drawings/baseline had some serious trouble being applied to bent old Lot 0 - 5 airframes initially in line for the remanufacture process. (Short answer: a lot of the holes didn't line up in major stuctural areas).
c. In due course, a deputy sec nav for acquisition reviewed the program and rightly decided to put it back into R & D. (A half a billion dollars or so after the R conception back in the 80's ... ) As had been previously suggested, the upgrade (without the wide chord blade, for various reasons) came out as new birds in the form you see it today. (Thank goodness!) More or less, a few details may be slightly off there.
Make the best of such money as is available. That's acq programs for real, versus theoretical / best case.
I ask you: consider how long the C-130 has been around, and the Huey.
Is there some pressing need to have a "new" helicopter?
I'll suggest to you that the industry, and the limits that physics / aerodynamics impose on helicopter performance have been reached, and any improvements are developed in marginal expansion of capability at significant cost differential.