Feel free to take any and all of my opinions with a grain of salt... but for me at least the SP404 mk2 might be in the same ballpark as the MPC, but it's not in the same league, and that's okay. Plus, for plenty of workflows the SP404 mk2 is quite good, and it can trump the MPC in specific use cases. The SP404 mk2 for example is great at fast sampling, it is incredibly portable, and the USB C audio integration is so dang simple and works so amazingly well that I probably put the USB integration at the top of my feature list for the MPC.
Where the MPC line shines is in sample manipulation and sequencing... this has been true from the start, and the line has progressed more or less steadily to present day. The current product line just continues to drive this point home with added plugin effects, instruments, and a very generous and consistent firmware update policy spread across the entire current product line. Sure, any of us can point to specific features and things that bug us, work weird, or don't find specific processes and workflows, but for me, the MPC is an absolute workhorse for getting tracks from start to finish, and over the last couple of years it's really blossomed into an excellent sound design tool. But to unlock any of the advantages and benefits here, you've got to put the time in. You can sample and flip content quite fast, but often the MPC drags you a bit "under the hood" as it were when you want to do something specific.
Where the SP line shines is in the speed of sampling and the immediacy of its effects... this has also been true from the start, and the line from my point of view hasn't deviated from this tradition. The SP404 mk2 has certainly added a lot of features that blur the line... and it is dipping its' toes into more sample mangling and sequencing, but I do feel like it is a bit disingenuous to say that the mk2 is going after the MPC line, there's a baby step there for certain, allowing for testing the waters, but the SP404 mk2 doesn't even really compare favorably against the MPC 500 if you are looking for an MPC style workflow. The SP404 mk2 is incredibly fast at taking an idea and flipping it, especially if you fit into it's idea of "how to do it", as long as you like the effects being relegated to 3-6 knobs, and learning them... you can really do a lot of damage. That said, the sequencer is still only functional, effect automation is okay, the midi implementation even with Type B being added is kind of a pain, and Pad Mutes just irritate me how it is implemented. All this could be me and my MPC workflow baggage, but there's just enough inconsistencies in implementation that I can see the light, but I don't know if Roland will get there or not.
All that said, I think the two really compliment each other very well. The MPC continues to be great for sequencing and sound design, and the SP continues to be great for very fast and efficient sampling and dropping in effects. Do they overlap a bit? Certainly... but I guess I have room for both of them in my studio, and am happy to keep it that way