I'm another one who thinks the concept is a beautiful one, but one that is clearly undeliverable.
I cannot disagree with you more - regardless of the cost I thought the whole concept was appalling.
I'm not sure how many of you live/work in London so are very familiar with the proposed location. I am and it is folly from start to finish.
1. The first requirement for a bridge, surely, is that it takes you from somewhere you need to be to somewhere else you need to be. The bridge failed on this primary requisite, joining a 'dead' zone on the north bank (Embankment area basically closed off by Middle/Inner Temple of the Courts) to a part of the South Bank without major destination (you walk along the South Bank, rather than travel to it). There are two bridges right next to the Garden Bridge (Waterloo and Blackfriars, and slightly further along Hungerford) that go directly from major points of interest on the north bank to similar on the south (e.g. Royal Festival Hall, National Theatre, Globe, Tate Modern). Contrast with Millennium Bridge which joins two of the most popular sites in London (St Pauls and Tate modern) and creates a new iconic view without destroying one (see below).
2. The iconic view from the south bank across to the skyline of the city and St Pauls cathedral would have been destroyed by the bridge by virtue of its height (way higher than all the other bridges due to the trees on it) and its location.
3. I love the idea of green links - I have one very near to where I work - but they need to be continuous with other green spaces, so the one near me basically joins to parks so they are continuous and you feel you never leave the park despite crossing a major road. Same with the 'High Line' in New York. This bridge failed on that count too as there are now equivalent green spaces on either bank for it to join.
4. In order to create the 'green' bridge dozens of fully established trees on the south bank which create a lovely shady area would have been destroyed - not very 'green'.
5. If a bridge is to be of any use as a transport link you need to rely on it being open - yet this bridge was to be closed on numerous days for 'corporate' functions. Without doubt the number of those days would have expanded over time - the notion that you might close a bridge for a private party is just bonkers, if it is to function as a bridge.
6. The local communities on both sides of the river were strongly opposed - tells you all you need to know - if they didn't think the bridge was needed then I think you can conclude it isn't needed.
I could go on, but I won't - terrible idea and great news that it has now been sunk for good - shame about the money down the drain too.