Can you explain exactly how you think wallets will "know" how to ignore
the invalid chain?

I'm confused - I already said this. For a fork to work, hard or soft, there must be support from a majority of the hash power.

Therefore, the usual SPV technique of following the highest work chain results in ignoring the minority chain produced by the hard fork.

BIP 101 is SPV friendly because the wallets would simply follow the 75% chain and never even be aware anything has changed. It's backwards compatible with them in this respect: they already know how to ignore the no-bigger-blocks fork that'd be created if some miners didn't upgrade during the grace period.

My point about IsStandard is that miners can and do bypass it, without expecting that to carry financial consequences or lower the security of other users. By making it so a block which includes non-standard transactions can end up being seen as invalid, you are increasing the risk of accidents that carry financial consequences.

That's incorrect: Miners bypassing IsStandard() risk creating invalid
blocks in the event of a soft-fork. Equally, we design soft-forks to
take advantage of this.

Gah. You repeated what I just said. Yes, I know miners face that risk, my point is that they do NOT face such a risk when there's no soft fork in action and have historically NOT faced that risk at all, hence the widespread practice of bypassing or modifying this function.

All this approach does is make changing IsStandard() the same as changing AcceptBlock(), except without the advantage of telling anyone about it.

> So I'll repeat the question that I posed before - given that there are
> clear, explicit downsides, what is the purpose of doing things this way?
> Where is the gain for ordinary Bitcoin users?

We seem to be in strong disagreement about which option has "clear,
explicit downsides"

Obviously. So please enlighten me.

How do ordinary Bitcoin users benefit from this rollout strategy? Put simply, what is the point of this whole complex soft fork endeavour?