Currently, at give away prices, Pompey have an average league attendance, including plenty of season ticket holders who don't actually attend, discounted tickets for UoP staff and students etc., of 17,892, making them the third best-supported team in League 1 (congrats on that by the way). Fratton Park's current reduced capacity is 19,669, and if they fixed the roof 21,000 people could watch Pompey lose in discomfort.
I'd be curious to see the feasibility studies which are going to convince Network Rail and the council to invest millions in advance of Pompey spending a single penny upgrading Fratton Park to a 30,000-seater turd bowl. Where exactly do they expect these hordes of new supporters to magically appear from?