The concourses at BMD have already been designed to be bigger than required for 53K seated fans, so as to allow for many more when/if rail seating is given the go ahead. In an ideal situation, for Covid, your capacity is driven by available seats, not by ability to move safely though concourse areas. Also, pedestrian modelling shows that it helps massively to have a number of check points to ease and maintain flow of people into stadiums, these cannot be implemented safely around Goodison due to the nature of the surrounding streets, but can be on the 'fan park' of BMD.
We need to also bear in mind that any build is going to take a coupe of years, by which time everything will be much clearer. If everything is back to 'normal' then we've not wasted any time, if everything isn't back to normal, then we still have a much better stadium in which to enact Covid regulations.