After getting 750D I just discovered I need to gave two versions of DPP running to be able to use both cameras
DPP 4 Won't open 5D files
DPP 3 won't open 750D files
What's the point???
The point is that DPP4 started a new code base (not a further development of DPP3 but an entirely new version), and they decided to slowly add the more current cameras to the cameras supported by DPP4.
Differences in CFA, noise profiles, features in RAW, lens profiles for DLO and other corrections, it all needs testing, development per model. So, with the new version they do not support years and years old cameras and operating system versions. They started out with supporting just a few current full frame models, and by now have expanded the supported cameras to a bunch of APS-C DSLRS, theentire M line up, the advanced serious compact line up.
They made sure that one can install DPP3 and DPP4 next to eachother on the same computer, even. Seeing that DPP is new a source of revenue, and the advances they made in DPP4, personally I think they do a pretty good job. They have made choices on the size of the software development teams.
So I can't process old 350D and 450D files in DPP4. DPP3 still does its job.
It is true that when DPP3 will not work anymore on new, incompatible with the old, OS-es, one can't use DPP to convert those old files anymore. Then one has to use a DCRAW based RAW conversion. The choice is, do you want an advanced RAW conversion with new features (what Canon chooses in regards to DPP, with its advanced DLO, picture styles and colour model) or keep on being based on the lowest denominator (be able to convert all RAWs without the new features).
Or of course, have a big development team to do both at the same time.
Canon chose the 1st, and I understand that choice, even though in some ways it is not ideal. Just how it is, though.