The regulars at the top of the table are there because they compete for the best players and pay the highest salaries. To make that possible they have to continue to attract large crowds that pay high ticket prices.
Saints are currently riding high with a great team, and I suspect the big spenders will be sniffing around in January. Players may or may not leave but Saints cannot expect to always find high quality replacements.
To maintain Premiership status long term I think it essential that Saints increase income from ticket sales and this means upgrading the grounds capacity.