"Pfizer's worldwide known erectile dysfunction product Viagra...

"Pfizer's worldwide known erectile dysfunction product Viagra generated around 500 million U.S. dollars in revenue in 2019."

You need to be careful how you use that statistic. For example if we think about potential market size for LTP and volume sales at say 20% of Viagra's sales, it won't translate to revenues of 20% of US$500mill because Viagra sells for around $80 per pill (from the recent LTP webinar) or $91 according to a Google search for prices in the US. From the same webinar, Spontan will sell for around $200 for 16 doses, i.e. $12.50 per dose.
So reaching sales volumes of 20% of Viagra's would not generate 20% of US$500mill, it would be 12.5/80 x 500 or around US$78mill or A$115mill.
However, who knows if will be 20%, 10% or 100% or more of Viagra's sales. Obviously the number will be smaller to start with and growing each year (hopefully very strongly).
That would be the opposite of what sales were doing for Viagra with its patents having expired outside the US in 2012 and were set to expire in the US in 2020 although some manufacturers were permitted to market generics in the US prior to 2020. Sales did stabilise post 2020 (second chart below).