All webfont licenses on MyFonts don’t require a subscription. There’s no monthly fee to use your fonts. Once you purchase the license, it’s yours.
In addition, webfonts you buy from MyFonts are self hosted. You don’t have to rely on an external service to keep your fonts working.
The vast majority of webfonts on MyFonts are licensed with a Pay-Once model. This means that you’ll pay one flat fee for your webfont license and never have to think of it again. The only time you’d be required to pay more is if your site traffic exceeds the monthly number of pageviews your license is set for.