Non-mining nodes might be viewed as irrelevant by the rest of the network but the relevance of running one should matter to you. It would also be in a Bitcoiner's self interest to do and validate his own transactions instead of relying for someone else to do them for you, which what SPV wallets do.
Maintaining censorship resistance and a sovereign asset system depends on it.
Censorship resistance, I think, is built on the rational mining incentive, not users running full nodes. Miners are incentivized to publish any/all transactions they can based on fee priority, because of the block reward. That's what keeps them from censoring any given transaction -- fees.
The problem with SPV wallets (aside from privacy) is that users can't know if a payment they
receive is valid. Whether a payment they
send is censored by the mining network is a separate issue entirely.
Are you saying that mining nodes can censor my transaction if I do it from my full node?
Sure they can. A malicious group of miners just needs enough hash power to orphan any blocks that contain your transaction from the best chain.
[I only (correctly) stated that online non-mining (full) nodes are irrelevant to the online process of consensus.
I believe this is correct. Non-mining nodes don't really participate in the consensus process. They merely validate what miners do.
But the existence of validating non-mining nodes is a powerful check on miners, since a robust network of full nodes will reject attempts at dishonest mining that break consensus rules. "Reject" isn't even the right word; they'll be completely ignorant of the invalid forks because they are incompatible protocols. Major economic nodes like exchanges are especially relevant here, because they won't risk accepting invalid coins from miners, which presumably keeps miners in line.
If a miner breaks consensus with economically relevant nodes, his future block rewards may end up worthless. That's a strong incentive not to break consensus.
They can turn your fork into mush that never confirms and has continuous chain reorganizations.
Many people predicted that would happen when Bcash forked. Instead, Bcash had to hard fork a second time because of slow block times.
Apparently, there wasn't enough incentive amongst miners to risk attacking Bitcoin. What makes you think that incentive will ever emerge?
They won’t spend anything. The attack will be very, very profitable. They recently created futures markets in their lairs which they control. Nasdaq futures being added soon as well. They do not have to spend anything to bribe other miners. They simply tell the other miners that they can keep all the P2SH SegWit booty which they confiscate in their blocks. Mining is anonymous. Every person you know who is mining will jump on the chance for free money, while they won’t tell you they defected from your unprofitable USAF. Money talks, bullshit idealistic bankruptcy walks.
For various reasons (like massive ventures with significant required infrastructure and high electricity costs), industrial miners are not anonymous.
As Peter Todd points out, colluding and openly attacking the network could bring significant bad PR or even criminal charges.
Segwit transactions represent 1/3 of the network. You think when 1/3 of the network has their outputs stolen, that all users are just going to quietly eat the losses? They'll download the miners' client and start following the miners' chain? Why?
UASFs / hard forks to address such miner attacks may be extremely inefficient. However, it doesn't follow that most users will remain on a network where the majority of miners have participated in thefts of this magnitude against its users. And those "other miners" you mention? They need to weigh their long term rational interest in a healthy network against the short term incentive of "free money."
This brings me back to the rational mining incentive I mentioned earlier. In your scenario, that incentive has blatantly failed. There's no point in switching networks to follow the miner chain if there is apparently no reliable incentive to keep miners honest. If that's the case, Bitcoin's value should plummet and this whole conversation is moot. (And that's the overarching reason why miners will probably avoid ever engaging in the sort of attack, unless at the behest of a powerful nation-state)