Although I expected (and want) Canon to come out with a more advanced model than the EOS R, rather than a lower model, I can see the business sense in a 'budget' model (being the EOS RP) enticing photographers to the Canon FF mirrorless / RF mount. With aggressive pricing, and an adapter, they could convert many existing Canon DSLR users (both APS-C owners with their EF-S and EF lenses, as well as FF DSLR owners).
I currently own a bunch of Canon photography gear, the most used being the Canon 80D with a mix of EF-S and EF lenses (including some L glass). I also own the M5, with 4 dedicated EF-M lenses, which I use regularly. The Canon FF mirrorless (RF mount) is something I expect to get into in the future... and being a middle aged guy, I look forward to many years using that. But I want to enter it at the right time with the right model.
The EOS R did not work for me (poor ergonomics, and lacking some features I want). The EOS R's EVF is amazing though... really night and day above the M5 (both in terms of size, viewing pleasure - particularly brightness, clarity, contrast, colour and dynamic range). Not that the M5's EVF is truly terrible, it's 'ok' - and certainly better than some EVFs I have used in the past. But the EOS R's EVF is just so much more pleasant to use. If the EOS RP has an EVF that is say 70% to 80% of the way to the EOS R's (compared to the M5's, I think it will be good enough for most people of this target market.
Other specs that I think are important in the market segmentation for an EOS RP model: sensor IQ at, or slightly above the 6D DSLR models, AF and subject tracking similar to the EOS M5 / M50, no need for amazing 4k video, but having that as an option (even with a 'crop mode' would be ok). Having an EF-S/EF to RF adapter available in the kit (or fairly inexpensively) would be important too.
The swivel screen, DPAF, Canon's colour science and compatibility with Canon's accessories and 'system' are a given... So yes, if it's priced right (my guess, initial RRP between USD $1500 and $1700)... and then dropping a hundred or few hundred after a few months, will mean it could sell quite well.