As has already been posted many times, the income tax system works well as a ‘free’ proxy for means testing anyway. In your over £100k example, 60% of the top slice, which might as well be the state pension, goes straight back to the treasury. The fact that they choose to do something you disapprove of with their after tax income is a distraction.
For almost everyone with pension income other than the state pension 20% of the state pension goes straight back in tax. How much more money do you thing will be clawed back, and at what cost of a complex means testing process?
Now, overhauling the tax system to make it simpler, and rolling employees NI into it (as the conservatives started) would mean than pensioners and those working would pay the same rate. That would be a good thing. Having a more graduated tax system, without cliff edges, would also be good. And could be structured to progressively take more from the higher paid, unlike the current perverse system that actively discourages anyone from working for between £100 and £150k. If we had a few hundred thousand more people paying tax in that bracket it’d make a material contribution to the treasury.
Having got rid of employees NI you’ve silenced the argument that ‘well. I’ve paid my NI so I’’m entitled to the state pension. Increase the auto enrolment % from its current inadequate level. Then dump part or all of the triple lock such that the state pension reduces in value over time, and is replaced by pension credit or whatever it’s called. Then you have a long term strategy for reducing the cost of the state pension, while giving people ample time to have made alternative provisions.