ok, fairly normal £100k single income family, 450k mortgage with 25 years left, which is probably even a bit low for a small 2.5-bed terraced or semi somewhere commutable to London (assuming ~550-600k purchase price). 2% fix comes to an end, rate is now 6%. Mortgage goes from £1900 to £2900 pcm. That's £12k per year more cost before you've even looked at energy increases etc. £12k per year is 20% of that entire family's take home, just from an increased mortgage rate. Where are they going to tighten their belt? They can't downsize. If they move further away, they won't be able to commute. Somewhere else in the country their job might not exist or might have a significantly lower pay that makes moving a net pointless effort financially, or could cost more if they rely on family for anything.
Remembering that all mortgages are limited to gross income multiplier, not net, the "issue" here scales fairly evenly whether someone is on 30k or 100k, except that the extra earning required to offset becomes far, far higher. Someone on £100k is not exactly rolling around in lots of money. Do you REALLY believe that a person who is doing ok but FAR from wealthy be subject to 60% punitive marginal tax on any pay rise they then get? It's COMPLETELY the wrong part of society to be targeting.
Am I saying that people on 20k / 30k aren't also going to face issues? No, nobody is, and of course, this is a **** show for them too. But their life is amazing compared to kids starving in Africa with no access to fresh water, so why should we care about someone who is still in the top 1% of global earnings? Everything is relative, just because someone else might be even worse impacted it doesn't mean that there isn't a struggle for somebody else that we should be trying to help mitigate
TLDR: someone on 100k in the south east is not rich. You're basically the only person arguing that the 60% marginal rate is a good thing and still fit for purpose 13 years after it was introduced without an increase. it's not even good for increasing tax take as anyone who can afford to uses pensions etc to avoid it, where as more people would be likely to take it as income if this was scrapped putting money back into the economy now when it's needed, and 40% of something is better than 60% of the nothing many people try and pay currently!