I don't think it is a slam dunk that the older Smith revolvers were "better" across the board. We just got used to traditional looks and design features, and devoted fans of products are typically resistant to change. Yes, I do think the external finishes were better. I do think a hammer nose requires a bit less mainspring force to reliably ignite primers than frame mounted firing pins. And I hate the internal lock just as most people do, so no argument that it was a step backwards. The J, K, and L frame guns have a less elegant sweep to the curve in the frame adjacent to the hammer as a result of the lock. But really, I think that's the extent of the old guns' advantages over the new.
The new guns have better metallurgy, better yoke retention, better rear sight mounting design, better front sight system (DX), better extractor design and orientation, frame and yoke mounted ball detents on some models, and less chance of getting misaligned barrel/front sight due to the 2 piece shroud/barrel design. I personally do not believe the MIM parts have caused lesser trigger pull quality, and I've not found the MIM guns to be any more difficult to tune for excellent trigger quality. Nor have the MIM parts proven to be "problematic" or any less durable. The MIM trigger and hammer have enabled design changes that eliminated the need for pins to contain the hammer stirrup, DA sear, and trigger rebound connector. The sear surfaces on the MIM parts are as smooth or smoother than the old case hardened parts because they don't have the tool marks from broaching that must be stoned out.
It is human tendency to always look at the past as the "good old days" of manufacturing. Was there really better QC and fewer problems in the past, or did people just not hear about quality issues in the past because we didn't have the internet then and now we do? Who knows?
I have old Smiths, new Smiths and in between. I've taken all of them apart. I haven't seen any evidence of the old ones being superior to the new ones in any way except for the presence of the hated IL on the new ones and a bit nicer external finishes on the old ones. I think we have just traded for different sets of "problems." I've never understood the arguments for why the 1 piece barrels were unquestionably "better" than the new 2-piece barrels, as there are advantages to both designs.