The A2B line has by all measures been a resounding success.
It opened up a large hinterland, six large towns now have 4tph in each direction to Glasgow and Edinburgh directly (double frequency with longer trains), new stations along the route, better commuting and leisure opportunities, faster journeys ect. Granted, it's a slog from Airdrie, but that's a problem that will eventually be solved, and nothing to do with the rebuilt link.
However more importantly it restarted the desire to reopen closed railway lines across the country.* We would not have seen Levenmouth or EWR (edited, alloa was already open and the borders was authorised before but opened after) so soon without the A2B to kick things off. It proved conclusively that rebuilding railway lines can be hugely beneficial, and that they are worth the investment in the modern era. It kicked off the current wiring in Scotland, and has helped serve as a test case for many feasibility reports, laying the groundwork for reopenings all over the country.
Yes, it has been amazingly helpful during disruption, as a capacity booster, and as a vital link for deprived communities, but it's legacy will remain as the bedrock of all modern reopenings across the UK.
*Obviously the robin hood line, Corby and scattered station reopenings occured throughout the period 1990-2010, but this one is different. This was the first modern railway rebuilt to modern standards along a closed track bed. A lot of the lessons that were learned here were used in the Borders, and continue to be used across rebuilt lines.