There isn't the facilities at most grounds to accommodate extra people, even at CoMS on the lower tier it is too busy in the concourse due to 3 rows of additional seats at the front.
imagine being in the away end at west brom with 50% more people there, or Bolton, Hull, Charlton, Blackburn, Wigan, M United, Liverpool, Everton, Villa etc Maybr Arsenal could cope with an increase, but most grounds are no up to it.
If introduced the numbers would not be upped, and less fans would stand according to WBA proposals.