Speaking as someone who was raised Muslim but who still celebrated Christmas each year as a secular holiday, I think YABU.
There will definitely need to be some level of restrictions over Christmas, and it seems the government will implement some thresholds for number of people/households mixing. It's unlikely to be a free-for-all.
Without some easing up, as PPs have said, there will be a higher rate of non-compliance which can extend beyond Christmas. Whereas if they lax the rules a little, then they can still prevent huge gatherings and maintain some level of order.
For some people, Christmas has been the only thing getting them through this year. I, for example, live abroad and the thought of seeing my parents and my brother's family (SIL, niece, nephew) at Christmas has sometimes been the only thing helping me power through. The country I'm in requires a negative PCR test to be taken before departure at the airport, so I don't see myself as adding any real risk to my family if I social distance whilst wearing gloves and a mask during the trip...
With correct management, mixing of households should still be relatively ok (e.g. allowing mixing between households who have self-isolated for a set number of days beforehand / or between people who have negative PCR test results etc.). The issue with this is, of course, that the government botched the whole track and trace process.
Christmas has a substantial economic impact which is incomparable to the other festivities you mentioned - the vast majority of people in the UK participate in the commercial aspect of Christmas, only a minority participate in Eid or Diwali - they have less economic consequences to being cancelled and less chance of causing mass civil disobedience/rioting in the face of cancellation.
Those people who have a moral objection to mixing between families over Christmas can follow their ethics and stay home whilst the rest of us take cautious steps to stay safe but enjoy our time with our families.
YABU to frame this complex issue as some sort of agenda against Muslims and Hindus / prejudice in favour of Christians, when really there are many more factors at play.