Very roughly, the government loses 30% to 50% of the purchase price (depending on marginal tax rates) in foregone income tax and NI. It wins 20% of the purchase price in VAT, and about 20% of 5% (1%) of the purchase price in corporation tax.
So the government is a pretty substantial net loser. And that assumes the bike wouldn't have been bought anyway - most people seem to be using the scheme to buy a bike they would have bought anyway.
And the very busy bike shops, importers etc that have had to employ more staff to cope with the increased sales. What about their income tax and NI?
And all the accessories bought? I've probably spent about £500 since last November on extras, and will probably continue to spend on other cycling consumables.
And assuming that the bike wouldn't have been bought anyway. I had a perfectly servicable BSO but I bought my bike precisely because of the C2W scheme. I may not have otherwise. I don't think most people would have bought the bike at all, unless of course you are assuming that because of the amount of people on this forum that have done so.
Now how are your sums stacking up? Very roughly.
Edit: And then of course we have those that actually realise they love cycling and then spend £x thousands on N+1 etc.