The original EGIP plan was for 6tph 6 carriage EMUs running the shuttle. That required major expenditure across the route, plus the loss of the Anniesland shuttles into Queen Street. It was then calculated that a more cost-efficient way to deliver the same benefits would be to extend platforms to 8 carriages without adding more services. The extra infrastructure components have been dropped, but there is nothing stopping them being constructed in future. Indeed, the two major enhancements planned - the Greenhill Junction works and the Dalmeny/Almond Chord - are still included in Network Rail's long term planning. Both schemes will likely go ahead in the 2019-2024 timescale.
If the rebuild of Queen Street had gone ahead without the plan to extend trains to 8 carriages, then it would have been prohibitively expensive to extend platforms later on. Therefore, by extending platforms first, it enables all the service improvements originally planned to be combined with longer trains for even greater benefit, while the other way around would not have allowed this. In addition, without the platform extension works at Queen Street it may well not have been possible to use shortened HSTs on InterCity runs, meaning that these would suffer too due to the use of D(E)MUs with underfloor engines.
As the Waverley Railway Act demanded that the whole line be built, or none of it, there is little that can be done to reduce expenditure other than reducing the amount of double-tracking. Would you rather have no railway at all, or a railway delayed even longer, or one which has actually been built despite being of dubious BCR? If you wanted to spend even more money on a gold-plated line that would have meant spending less money on some other worthy cause, and the role of a government is to make these sorts of hard decisions. If you can get 95% of the benefit for only 70% of the cost (for example), then that cost saving can be used to spend money on other schemes to benefit the country even more.
GARL was a simply woeful scheme which made no and still does not make any sense whatsoever. Most of the business case was to do with the wider Paisley Corridor improvements necessary to deliver it, and those works still went ahead. The expensive viaduct over the playing fields and the M8 would only have seen a tiny number of passengers. Instead of being empty after Paisley Gilmour Street, these trains now continue on to where people actually live, providing a better transport system for the vast majority of people in Scotland. If you want to improve the accessibility of Glasgow Airport for people in Glasgow you would be better off funding better bus links from across the city, rather than providing a shuttle service into the city centre where very few people actually live. The few passengers who do need to go between them can be dealt with more efficiently by buses, taxis or some other transport system which can justify its existence with other passengers doing other journeys. A tram line from the city centre to the Airport via the hospital, Braehead and Renfrew would be a much better use of money.
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