The evidence on new, modern stadia is quite clear:
- Broncos average crowd at Brisbane's old shithole ANZ Stadium slumped to 19,700 by the time they moved out to the rebuilt Suncorp Stadium, then their crowds grew year-on-year to a peak of 35,000 in 2010
- Melbourne averaged 11,900 at Olympic Park when they moved out and their crowds have grown year-on-year to a peak of 16,100 in 2019
- Parra averaged between 10 - 15k for most of their recent history with the odd exception, their new stadium delivered an average of over 18k in the first season which would have been higher again this year if not for COVID
Cowboys will follow suit, no doubt the Roosters will get a bump in crowds at the new SFS
Sydney's ANZ is not an example of a modern, purpose built RL ground but even the permanent tennants in this imperfect example have had a bump from playing there despite all of its drawbacks:
Souths average crowds before moving to ANZ were around 10k most years with a few exceptions. Since 2007 at ANZ they have averaged either 15k or above in all but one season.
Bulldogs average crowds before moving to ANZ were around 10k most years. Since 1999 at ANZ they have averaged around 15k or higher every year except 2000 which was 12k.
This doesn't include the evidence from AFL having upgraded grounds and having bumps in crowd numbers as a result.
People say they want the nostalgia of the suburban grounds but there is overwhelming evidence to suggest that if you build a moderrn, purpose built facility people will attend in greater numbers.