If bus and rail services were treated as a unified network as they are in most countries with decent public transport, then the rational decision would be to pay for a decent bus service rather than spending a huge amount on building a railway.
That's completely fair, and that was what the BTC was theoretically all about. But Brackley could have a case for reinstatement if there are sufficient houses built along the old GC route to Rugby - and the railway would have to go in ahead of the houses, obviously.