That is a feel good statement but is very shallow in actual application. "those who are doing it" implies success and we are not talking about that group. It would be more accurate to say:

"Those who say it cannot be done
should not discourage those who are attempting to do it."

So that is closer to our conversation but then it is flawed. My kids want to attempt things all the time, but that doesn't mean it can (or should be) done. It sounds nice but there are somethings in life that are just bad decisions no matter how bad you want to "attempt" it. I get that this might sound negative. However, there is a reason why we do research and planning before just going with our gut.