It's not actually about hating pensioneers, it's about the dislike of A) Several benefits creating needless bureaucracy when you could merge them in one pension. and B) Of "wasting" money by giving those benefits to people that could afford their gas.
For starters, it's not solely for pensioners. It's a supplement to a range of benefits, including Job Seekers Allowance (JSA - unemployment) and Employment and Support Allowance (ESA - disability). Having a one-time payment piggyback on these other benefits isn't a huge bureaucratic cost, given it's just refer to a list of those eligible once a year and make the additional payments. Frankly I don't see any difference even if you rolled it into the separate benefits as three different payments instead of one core one.
And yeah, keeping the benefit separate allows it to be tuned to increases in fuel costs or other economic realities. Except that in reality it just means it's an easier target for cuts. You can cut the winter fuel allowance by £100 a lot easier than cutting an annual pension by £100. So I guess I don't care as much about it's existence as individual payment and more it's continuation as a payment.
As far as means testing goes, it's for people already on a benefit that mean they are on a limited income. Given that that's by far the easiest means test out there, additional testing to see if they can 'afford' gas each winter would mean a pretty detailed audit of their place of living and local living costs (often the winter fuel allowance also pays for increased food costs), and any savings or other assets. That would be a significant expense for
every eligible person. You would need to deny the payment to a pretty large number of people to make up for that added cost, even before making any additional savings.