I think we need a dose of realism.
As I understand it, the project was a whisker away from not happening when the final estimates were compiled, as it was much more expensive than anticipated. As someone who has (and still does) build railways, the extra cost of a wider bridge is more often than not another couple of hundred tonnes of concrete. Often it means more land, additional highway and utility works, additional environmental mitigation. Had the railway been built with double track bridges etc. it would easily have added tens of millions, perhaps a hundred million, to the cost. That would definitely have killed it.
Further reality, which maybe uncomfortable for some readers. If easily offended by economic reality, please look at another thread.
The line had an appalling business case before it was built, and still does since it has been built. It will not pass any post implementation evaluation of benefits realisation. Scotland as a whole (and by extension the UK) is worse off than it could have been without the Borders Railway as a result. This is because the £400m+ could have been better spent elsewhere to realise greater benefits.
There are literally hundreds of railway projects across the country, many in Scotland, that have better (much better) business cases when assessed using the same methodology as Borders, but are not happening because there is no money. That sort of money could have transformed the entire West Yorkshire rail network for a couple of million people, or Bristol and Avon's, or a number of other places. And that's before we come to non-railway projects, or non transport projects. That sort of cash could build a new super-hospital that would transform the healthcare provision for an entire region, whilst significantly reducing the cost of that provision.
Clearly the Borders railway has been good for the communities along the line of route. The same could be said for almost any community that gets new infrastructure. However that doesn't make it right, if the decision to proceed prevented other, better projects from happening.