While I agree with you that you'll need to increase the time to see real benefits it's also not true that you can't go more "all out". As you get stronger you'l

While I agree with you that you'll need to increase the time to see real benefits it's also not true that you can't go more "all out". As you get stronger you'll be able to peddle harder, your peak wattage will go up, and you'll have effectively increased the intensity compared to where you were before. Now obviously if this is the only exercise you're getting you're still going to hit a ceiling pretty soon.