The thing is, if you build the M6 bypass from general taxation, then you
are effectively subsidising those who use it. You might argue that
motorists
at large will pay for it, and motorists at large will benefit from it, but
in
reality it is only a small group of people who will benefit from it, yet
if
it were paid for through general taxation, a large group of people would
be paying for it.

That's the whole point of the state paying for things. The point is that
overall everyone gains equally. The BNRR in the Midlands, M25 widening in
London, M74 in Scotland, etc.

The Government should not be subsidising things that bring a high
benefit (and have a high cost) to a small number of people.

In that case we should adopt the American model of very low taxes but
nothing provided by the state. Why should I be subsidising the NHS when I
haven't used it in 10 years? Same argument.
The answer being that I might need to use the NHS tomorrow, and you might
need to use the M6 Toll tomorrow.
It benefits everybody and the economy as a whole. If a lorry carrying goods
from Felixstowe to Manchester gets stuck in M6 traffic jams for hours,
businesses in Manchester lose out. It has a national impact.

Besides, if it weren't going to be a toll road, it would fill up with
traffic and be no better than the normal M6, and we would be back
to square one despite having spent £900 million.

It would fill up as-is, yes. What they should have done would be to build it
with no local junctions - just access from the M6 and M42 at either end.
That way, only long-distance traffic uses it, there are no new business
parks or shopping centres built next to it, and it serves its purpose as a
long-distance strategic route.