> If there is not a significant influx of new marriageable people then over time the marriageability of the pool will decline

That seems to be extremely unlikely, people have finite lifespans and are only in the marriage pool for a small fraction of that. More importantly your website could easily be targeted to an even smaller pool say 25-45 and ignoring deaths and divorce your already ~10% turnover per year if you own 100% of the market. Actual numbers depends on what percentage of the pool starts married, becomes a widow etc but their’s plenty of new people to make up for any couples. Further, happily married couples are great advertising.