Some competitions have rules where if you win a backyard GC you have to go pro the next year. But of course that only applies to certain competitions. If they don't have those rules then you can stay backyard as long as you want.
But your question is based on what other teams may think. In that case you can use the do you want to be a "big fish in a little pond" analogy since you seem to be doing quite well in backyard or give pro a shot. All depends on what you want your team to be and what your goals are in the long run. If you just want to have some fun and only cook a few comps a year then stay in backyard.
Other factors come into play as well such as additional cost. Which have been noted above. For me it all boils down to what do you want it to be? Do you want to see how you stack up against pro teams or do you like the generall fun of competing and hanging out with your friends regardless of who you are against? And there can certainly be other reasons to compete. None are right or wrong.
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