Actually I dont think the 'looking pretty to attract people' notion is stupid, but that's another discussion. Leave that aside.
It's not 20 shops around the San Siro that we are talking about, that already exists. In new stadiums, the model is to build an attached shopping center with ample branding space, within the stadium. The superstructure will have to be expanded for this, and access will have to be tremendously improved.
Aesthetics is hardly an issue here actually - People are attracted to new stadiums because it's comfortable going there, they are safe, and they give you a more complete experience - which includes shopping, dining and so on.
San Siro has enough aesthetic appeal to cut the threshold as far as that is concerned. That is why I said the priority should be to provide the experience rather than improve the looks. The facade is good-looking already. Which is why it will be much cheaper to renovate the stadium.