The stadium capacity argument has rumbled on since those first consultation phases.
It has always been about juggling a set of simultaneous equations relating to both supply and demand, the price-elasticity of demand for our fanbase (plus any greater latent support out there), plus size and value of local corporate demand..... all overlayed by an almost exponential cost/capacity (Supply) curve.
The general rule of thumb often quoted in stadium design guides, is that average cost per seat increases dramatically with increased capacity..... sometimes doubling (or more) construction costs for every 10k increase. Of course there's a lot of wiggle room in that. It is also very much dependent on the design format chosen, structural and other complexity, cost of land acquisition, planning and preparation, quality and scale of outfitting and amenities etc. The resultant cost then has to be measured against the ticket-price/demand curve to hit the so called sweetspot and to maximise returns to cover those costs.... and ultimately increase the club's coffers.
When West Ham moved to the London stadium they had to cut their ticket prices to fill it.... so much so, that their matchday income barely changed despite almost doubling their capacity. In recent times that capacity has increased slightly and the matchday income has began to sneak up too. But they were able to stack'em high and sell'em cheap because they got the stadium for effectively free. Unless Moshiri is gifting us the stadium, that does not apply to us.... far from it in fact. Similar for the larger clubs who chose redevelopment.... their expansion costs have been significantly lower and ticket price increases have been minimised or not been necessary. When building from scratch there is no existing (free) capacity to subsidise new (expensive) capacity...... so you are less able build speculatively and not have to worry about filling every seat for every match. The large Spurs and Arsenal stadiums used multiple high-quality corporate tiers with layers of executive boxes to substantially offset those costs.... (some might say detrimentally in terms of atmosphere). Apparently there are only 20-22 boxes at BMD, the corporate offer is much smaller and isn't in its own dedicated exclusive tier or 3, as at those stadia.... so a larger proportion of any increased costs would have to be met by general admission tickets.... so, it isn't just a case of aiming as high as possible to compete with our traditional peers or future proof our club (however important that is too)..... It is quite a cost-critical balancing act, especially if the sugar-daddy naming rights is less lucrative than first thought or as elsewhere.
IMO, the original 60k+ stadium was almost certainly dependent on a massive cash injection from a successful Commonwealth Games bid. When the Durban games were cancelled, the time-frame changed and Brum won, probably on the strength of being the more prepared safer bet. There was a long silent spell after that decision. When the club eventually made it's next stadium announcement, the plans had been rationalised to the smaller capacity. I don't think it's any great mystery why. So much for the consultation process, where the 60k+ option gained the most votes?
I've always had my doubts about the stadium costs. Owners often beef them up to exaggerate the quality (and probably boost any sell on value), and to temper the fans desire to "go big".... so that they can control prices by limiting supply, as at Juventus. At the time of the proposals Feyenoord were quoting £400m for a structurally more complex 63k, triple-tier stadium, with closing roof and also on partly reclaimed land. Moshiri's own design for Roma was quoted as €300m for the same capacity, but again in many ways a structurally more complex design.... so, it was slightly difficult to see how our slightly more structurally basic 53k stadium was costing so much more. That's before the owner said the costs had risen by 50% post covid, which the club itself now denies.... of course project and construction costs are different, but often conflated to misrepresent.
Some great discussion about smaller meaning more intimacy and better atmosphere etc. That can be the case. Tight smaller bowls with lower roof lines generally give greater unity and accoustic enhancement than tall high roofed multi-tier stadia.... that fragment the more vociferous support etc. There are contradictions to that though, with the likes of the millenium/principality stadium in Cardiff being multi tier and probably more atmospheric than any other stadium in the UK or the San Paulo/Maradona stadium in Napoli San Siro, Milan etc. It is very much dependent on layout, proportions, roof geometry and proximity. The comparison between our 60k and 52k designs are not massively different in those design terms. It is literally just adding another few rows all around. If anything, there is probably slightly better accoustic catchment in the bigger version. Of course, much would then depend on if it was full or not.