why not? sure there is a chance that it will get lost or stolen (same as checked luggage) but if you seal it up in a box how is a thief going to know if its $4000 in gear or $4 in teddy bears?
I have flown many times with my leathers, boots and helmet as carry on with out issue. Other than I got off the bike in PDX and flew to ATL after an endurance race and a guy bitched the whole flight about the smell of race fuel i have had no issues. That being said when I was racing all over the country I usually flew 1st class because of work frequent flyer miles..
We had two helmets in a gear bag, one in a soft sided bag, one "hard" case... they both traveled fine across the Atlantic as a checked bag. Even over size and overweight...
I flew a lot to races all over the country with all my gear in cargo, never had problem. Most time I found they opened it to see what is in there. Neck brace was usually what they cant tell what it is from scan
Race gear, Ferrari's, diamonds, etc etc etc...everything got shipped from somewhere at some point. I started racing in the D-Air suits back in 2012, several years before they were available in the US. I had to get all of them shipped from Italy. Never had a problem. If it was going to be a PITA, I wouldn't hesitate to ship it over there. But I would ship it overnight, a couple days before I actually needed it, leaving extra time in case of a delay.
Just be super nice to the flight attendants and they won't give you a hard time about stowing a slightly oversized item. 90% (probably more than that) of the people you meet who stand to give you a hard time (cops. TSA. customer service reps) are just normal human beings trying to make it through their shift at work - just like you. "I know you have a lot going on, and you can't give everyone special treatment, but my son's motorcycle helmet is an inch too wide for the overhead bin, and we think it would be unsafe to have checked it. Is there anything we can do to work out some other arrangement that the FAA won't have a problem with?" Then smile. pat your kid on the head. Say thank you. multiple times. Maybe I'm just a sucker for good manners, but motivating people to WANT to help you is usually pretty easy.