The problem is that at an income level there's only a requirment to complete a self assement return if your income is more than £150k, which will probably exclude the vast majority of pensioners. Then there's the fact that the DWP and HMRC aren't even joined up enough to tax a state pension at source - the tax due is recovered through the penisoners tax code appplied to their private pension. So, by doing nothing (the current process) HMRC are getting 20/40/60/45% of some or all the state pension back from those with other pension income.
Politically simpler to break the link between contributions and entitlement by getting rid of NI, make people save more and gradually erode the state pension surely? With then beneidt that it gives people time to plan and doesnt leave one cohort disadvantaged.
On discouraging people from earning more, the impact is real, especially in heathcare. There are large numbers of doctors for example who actively keep their hours down so as not to trigger either the 60% bracket or the pensions trap, and because of the stress of the job I am sure. 90% of GPs work less than full time hours for example. Even if they are bit better off (though 62% tax and no child benefit make it maginal) the additional time at home is worth more.
That's mirrorred at a lower level across many other jobs and sectors as well, and probably impacts on productivity across the economy. Fewer people in work, fewer people working full time and low unemployment does not drive productivity or growth.