Horses for courses, some people will set small goals that are achievable and give a small reward, others will set a massive challenge and get a big satisfaction from it... if they achieve it. If you don't try you won't succeed, but trying does not guarantee success. There are some that know they will never achieve their goal, but enjoy the pursuit nevertheless.

The fact is that modern machines are so complex they require more knowledge than one guy can muster, certainly if building from scratch. The people you see running their own gas turbine are usually people with professional knowledge and experience.

The internet is a often a bear-pit, but you are open to public comment and everyone has an opinion, it is usually the naysayers who are motivated to post. There is something adversarial in human nature I think. Perhaps in some places most people say "yay, great idea, go for it!", while encouragement is nice technical criticism is also useful. If you say "my goal is X", then people are bound to point out the feasibility or otherwise. And I think most people feel obliged to warn someone if what they are doing is doomed to failure.

If you want to just do your thing, that's fine, but if you seek expert advice, listen to the advice. Filter out the purely negative points, build on the positive suggestions. It is entirely reasonable to suggest practicing on smaller, achievable projects, than going for the "big bang" approach. Whether in business, science or engineering, pretty much every success story has a history leading up to it.