Firstly, you don't get benefits 'in your own right' when you are part of a couple. Benefits are based on total household income.
Under the changes, the pensioner will be classed as a 'dependant' of the working-age partner, so not eligible for many top-ups available to single pensioners. No cold-weather payments. Liable for bedroom tax (or rather the younger partner will be, officially, but the result is the same).
Living alone, this pensioner, if their income was low enough, would get their income made up to a certain level by pension credit, and get some or all of their council tax and rent paid income.
If they have a younger partner, this will not happen. If their younger partner is looking for work, the benefits paid will be so low that the couple's total income will be less than that of the single pensioner mentioned above.
Of course, it could be argued that pensioner benefits in general are too high, and they should all be reduced, but I doubt that any single pensioners relying on pension credit will agree with that.
My point is that these changes are very unfair on the older partner (remember, there are many couples with a large age difference, where the older partner could be very vulnerable).