I agree with you on a lot of things you say, but I think you're totally wrong on this, on pretty much all of it, especially on backwards compatibility.
Especially since ARM chips support legacy stuff that goes back a very long way already ... they simply haven't dispensed with that stuff. The difference is that they have continually streamlined and made more efficient what they have, x86 has just become more and more bloated and MS have made this much worse by STILL not having a decent hardware / software identifcation and driver-fetching platform, a la many Linux distributions.
The problems of making different platforms that you identify are not problems at all. There will be literally hundreds of companies designing these in future. All of these will be designed within certain parameters and for the most part compatible. ARM provide reference designs and the basic SoC / CPU architecture designs. There are already working platforms with PCI slots etc for ARM, if you didn't know.
I just don't see much of a market left for x86 in the mainstream in 5 years time. There's absolutely no way they can compete on a cost basis (ever) and very little prospect of them competing in performance per watt.
ARM has support for older stuff, but the reason X86 is so bloated is os's will run on ridiculously old hardware, and its not so much arm itself, its the hardware arm gets used in. Right now its in, essentially non upgradable throw away devices, if ARM moves up into higher end laptops and "real" computers, because thats the argument here, if ARM starts to encroach into real laptops/pc's, then the hardware platform will change, and the way pc's are sold is insanely different to the way a closed phone device is packaged, sold, and thrown away and replaced 2 years later.
When Arm needs to work in a full size laptop, with 4 sata ports, and usb, and network, and with a hdmi/display port out, and mice, and printers, and, etc, etc, etc.
When they introduce a video out like, say they add in display port out on tablets.. what happens when in 4 years, display port is ousted for, some new output, thats not compatible, well know ARM will have to add support for the new output, but keep display ports going.
This is ultimately what is holding back "real" pc's, serial ports, parralel ports, vga, dvi, hdmi, display port, IDE, PCI slots, etc, etc, etc.
ARM right now has none of those issues, to long term move into desktops it will have to add all that stuff, and when it does, it will have to support those things for years long after they are all but obsolete.
Right now ARM apps are small with limited usage because they are used on limited devices, they move up into "pc's" and, people making apps for ARM based pc's will have photoshop working on software that costs millions to make it support it, and big changes become problematic.
Right now, as ARM is, its incredibly unlimited by all that, IF it wants to push forwards in performance up into AMD/Intel higher end cpu area's, it WILL need to support all that kind of stuff, the entire platform will change. Realistically ARM will need an altogether separate architecture, for high performance that doesn't impose any limits on mobile phones and tablets.
Theres loads of complications both in doing that, and the fairly simple fact that at some point people will decide not to spend £400 on the latest iphone, when, while it link up to a screen at home and give them 3d high quality gaming, they can get a £40 phone by then that does everything they actually need a mobile to do.
its not there yet, and tablets/netbooks will likely grow for a few years with arm devices, but eventually people will stop paying through the teeth for power they don't need at all.
Essentially right now, someone makes angry birds, its basic, small, easy, simple and will work on a basic platform, what happens when people want to buy a higher performance pc, want to use photoshop and Adobe don't want to spend millions making sure 15 different ARM designs, and every previous generation all work with their software flawlessly.
