Hello! My thinking is this:
RP clearly is a 6D2 successor
Except it's not. The RP was a new class of budget FF digital camera. The very first from Canon. The 6-Series has always been mid-grade and the 5-Series evolved from upper mid-grade with the 5D Mark II to pro-grade with the 5D Mark III (which was essentially a non-gripped 1Ds Mark IV).
The EOS R was a mirrorless 6D Mark II with a 5D Mark IV sensor. That's how Roger Cicala described it. Insisting the EOS R was a 5-Series body because it has a sensor formerly used in the 5D Mark IV is about as ridiculous as claiming the R6 is a 1-Series body because it has the same sensor as the 1D X Mark III.
There's probably an argument that the R8 also exists to placate those who used to buy the x0D series as well. Maybe it does double duty?
In that analogy the RP is the FF 77D. (The 77D is slightly more camera than a Rebel, slightly less camera than an xOD, the RP is slightly more camera than a FF Rebel, slightly less than a 6-Series.)
Having owned the 80D (well, it was my wife's and now my son's), I felt it was a fun, geeky, and relatively affordable alternative to the much more expensive full frames: it gave the "feel" of an "upscale camera" as seen in the hands of the well-heeled out on holiday, like that stranger who steps onto a float plane in Alaska with their 5Dx amidst the sea of people clutching their Best Buy bodies.
Maybe the 7D Mark II, because when looking through the viewfinder it looked like a 5-Series, or even 1-Series VF, and it did feel like a 5-Series camera when shooting with it.
But no one "stepping onto a float plane in Alaska" is mistaking a 7D or xoD for a 5-Series body "amidst a sea of Big Box store bodies." (By the way, plenty of people bought early 5-Series bodies at Best Buy. I bought a 50D body at Best Buy once upon a time when I needed it faster than FedEx could get it from B&H to me.)
Canon kind of lost that feel with the R transition, I think, although cameras like the R8 are probably their attempt to at least slot in a price equivalent. I don't think the R8 spiritually captures what was happening, but it sits at a good price point in the catalog. They'd do well, I feel, to make a mini R5 body in crop form and re-introduce the X0D series as an X0R line -- and that's where the new video functionality they've been pushing for the new gen would fit very, very comfortably for people like my kiddo and his friends. But I agree, it also fills that 6D character as well -- competent, trade-offs, but serves. Someone mentioned earlier here that the R6 is kind of the 5D of yesteryear and the R5 is something new (or maybe the new 5DS/R?), in which case the R8 is now definitely the 6D equivalent.
The transition is a lot clearer if one stops insisting that using a sensor from an older camera means the newer camera is in the same class.
Lots of Rebels had the same 18 MP sensor introduced in the 7D and the 18 MP Dual Pixel AF version introduced in the 70D. The R6 had the same sensor as the 1D X Mark III. No one insists that a Rebel Tx/x000D with the same sensor as an older xoD or xx0D is in the same class. No one insists that the R6 is a 1-Series body, even though the 2020 R6 runs circles around the 2009 1D Mark IV in every conceivable way (except maybe build quality). The limits of technology expand.
The tiers at Canon have always shifted a bit to react to market conditions.
The 20D-30D-40D-50D split off into the 2009 7D, which was a half-tier upgrade, and the 2010 60D, which was a half-tier downgrade at several points from the 50D.
The FF 1Ds series evolved into the non-integrated grip 5D Mark III when Canon gave the APS-H 1D series an 18 MP FF sensor and
claimed they were unifying the 1D and 1Ds into the 1D X. It's no accident Canon's highest resolution body at the time, the 21.1 MP 1Ds Mark III was discontinued at the same time the 22.3 MP 5D Mark III was introduced with a pro grade AF system. The 2008 21.0 MP 5D Mark II was intentionally slightly lower resolution than the 2007 21.1 MP 1Ds Mark III. The 5D Mark II also had a consumer grade AF system. The 5D Mark III used the same PDAF array part number as the 1D X.
If the EOS RP had any precedent amongst Canon's DSLR bodies, it was the one-off 77D. More camera than a Rebel. Less camera than a 70D.
The 6-Series is pretty straightforward: 6D → 6D Mark II → (EOS R) → R6 → R6 Mark II → R6 Mark III.
Sure, each model in the 6-Series has better AF than the previous body. So does each model in the 1-Series. So does each model in the 5-Series. So does each model in the x0D series, in the xx0D series...
The 5-Series is a little more nuanced: 5D → 5D Mark II were consumer grade FF bodies. 1Ds → 1Ds Mark II → 1Ds Mark III were pro grade gripped FF bodies. In 2012 those two series were merged into the pro-grade but non-gripped 5D Mark III → 5D Mark IV → R5 → R5 Mark II.
As time marches on lower tier bodies have always gotten trickle down from previous higher tier bodies. But as the lower tier picks up capabilities and features previously only seen on higher tier bodies, the higher tier bodies pick up new capabilities and features that didn't previously exist in any camera.
Only Canon knows for sure.
