Regarding the PC ecosystem vs the ARM and MIPS ecosystem - I'm not sure what happened back when MIPS and ARM made chips, just that most of their volume came when they were IPs, and at that point chip makers were keeping things rather closed. I'm pretty sure both Intel and say MS do not see the PC as the ideal model for their business, they just happen to profit more from it than they would from an attempt to kill it.
Generally, today ARM will sometimes break OS-level code between releases - something Intel would never do - and I think ARM does this because it sells an IP, not chips, and so in any case the only way to sell a CPU to a consumer is through a chip maker licensing ARM and then porting OSes to the new chip - and the chip maker can handle ARM-related breakage while they're at it. But it does not follow that ARM is to blame for most chips being closed and/or hard for an OS unsupported by the chip maker to work on the chip across its various generations; I'd guess that most of the difficulty comes from the chip-level breakage and lack of documentation, not CPU-level issues of this sort.
So IMO it's not that ARM or MIPS actively prevent(ed) the emergence of an open platform like the PC while Intel was pushing for open platforms; it's that Intel happens to have succeeded mainly selling chips for an open platform, while ARM and MIPS happen to have succeeded mainly selling CPU IP for various closed platforms.
Regarding the SPARC IP that you mentioned - I'd have to look at it more deeply to have an informed opinion. Generally for a chip vendor who wants source code, as opposed to wanting an open source IP, MIPS is just fine, they'll give you the source. Otherwise what you want from your CPU vendor varies (ARM for instance gives you not just the CPU but everything you need to build say a cellphone application processor - probably not the best one on the market, but a completely functional one. At the level of the CPU itself, you might want a bunch of things - like hardware virtualization support, or cache coherence with other processors, etc. etc. - that different vendors support differently, if at all.) So as I said, I don't think I have a serious opinion about the product you mention.
Generally, today ARM will sometimes break OS-level code between releases - something Intel would never do - and I think ARM does this because it sells an IP, not chips, and so in any case the only way to sell a CPU to a consumer is through a chip maker licensing ARM and then porting OSes to the new chip - and the chip maker can handle ARM-related breakage while they're at it. But it does not follow that ARM is to blame for most chips being closed and/or hard for an OS unsupported by the chip maker to work on the chip across its various generations; I'd guess that most of the difficulty comes from the chip-level breakage and lack of documentation, not CPU-level issues of this sort.
So IMO it's not that ARM or MIPS actively prevent(ed) the emergence of an open platform like the PC while Intel was pushing for open platforms; it's that Intel happens to have succeeded mainly selling chips for an open platform, while ARM and MIPS happen to have succeeded mainly selling CPU IP for various closed platforms.
Regarding the SPARC IP that you mentioned - I'd have to look at it more deeply to have an informed opinion. Generally for a chip vendor who wants source code, as opposed to wanting an open source IP, MIPS is just fine, they'll give you the source. Otherwise what you want from your CPU vendor varies (ARM for instance gives you not just the CPU but everything you need to build say a cellphone application processor - probably not the best one on the market, but a completely functional one. At the level of the CPU itself, you might want a bunch of things - like hardware virtualization support, or cache coherence with other processors, etc. etc. - that different vendors support differently, if at all.) So as I said, I don't think I have a serious opinion about the product you mention.