(OOC: No offense, but I'm going to ignore your direction here. The point of this thread is "what if Apple doesn't switch to x86", not the specific vendor of x86 processors.)
Well, the G5 debacle definitely cost Apple some precious time. A lot of PowerBook G4 users abandoned the platform when Apple ended up delaying the "next-generation PowerBook" until they could figure out the next generation with the AIM alliance. I don't necessarily think that the Intel switch would have avoided the specific problems with NVIDIA as a vendor in the G6 models-- that's the big reason why the modern PowerBooks run AMD chips, which is actually quite cool because now that POWER-based non-Macs are starting to gain traction, it makes things so much easier to do with cross-compatibility, and is a big reason why Vulkan and OpenCL has seen mass-adoption by Linux and Mac users vs. the awful CUDA core nonsense that has kept NVIDIA plagued with problems for years!
(And that isn't to mention the fact NVIDIA hasn't made a decent GPU since like, 2012, but that's another story entirely...)
I recall a Marklar build dropping on BetaArchive back in the day, but it was basically just Mac OS X 10.4.7 ported intentionally to Intel's monstrocity, the Pentium 4.
On that note-- in the long-term, the thermal and performance differences between the x86 and PowerPC processors really wasn't as severe as some press outfits liked to pretend it was. The Intel Pentium 4 and successor platform, Pentium 5, were... plagued with very high base TDP, lower clocks, and general malaise. I remember seeing a side-by-side comparison of a Pentium 5 high-end enthusiast custom build vs. a Power Mac G6... and the Pentium got absolutely bashed by the G6.
AMD wasn't much better once they started falling behind in the IPC battle. Remember the Athlon X6 and FX chips? Yeah. Big jokes, particularly when you needed to do productivity. It's the main reason why POWER8 and POWER9 have taken off so much among PC enthusiasts, particularly since the new CEO of Apple has opened up to clones again. It's easier and more efficient to simply buy a PowerPC motherboard, CPU, and a copy of Mac OS X than it is to try to finagle the horrid x86 family of chips.
If Apple had switched to Intel, I don't see the PowerBook, Power Mac, or iMac families being anything more than glorified Wintel PCs. Apple may have tried to stop users from running Windows through some lockout, but honestly... once the genie's bottle is open, people are going to go with what's got the most compatibility, and Mac OS X is still quite behind in that regard, even with the release of Klamath (10.15) just a few days ago-- but the adoption rate is promising, particularly since Apple's stepped up on the UNIX compatibility front.
In fact, I just ordered my own PowerBook Pro yesterday-- the new 2019 model looks really good. 8 cores, 3.6GHz base clock, 16GB DDR4... a nice replacment for my 2015 PowerBook Pro. The quad-core POWER8 is just... a little long in the tooth, I think, what with new improvements to the production and IPC.