erm... Hate to throw fuel on the fire - but a triangle is
most definitely the "strongest geometric shape" w/o a doubt or debate - arches do come in a close second, the only problem being it's heavily dependent on where the load is... and a bike frame isn't ideal loading for your typical arch (thus the added supports in a cantilever).
BUT - it's really a moot point, obv cantilever cruisers are more than strong enough for our applications as has been proved by thousands
of our members. The only
relevant concerns applicable to this thread would be alloys and quality of welds, even gusseting. If the frames had a reputation for breaking in the middle of the arch - then yes, it'd be the a geometry problem - but they don't, they break near the welds... which points to a different problem entirely.
In the end, the above isn't if a Cranny needs be "beefed up" because it's a cantilever - when it comes down to it, it's a budget bike and that's
the only problem. I'd no sooner trust a budget diamond frame than a cantilever as it's not the geometry that is the root of the problem, it's the lack of quality materials and welds evidenced by actual failures and not just theory.
Given the above - frame style
becomes irrelevant as a strength consideration - it's purely personal preference & that has no right or wrong.
If ya wanted to talk about maximum performance
- like power to weight, now we get into minimizing materials, in the interest of the ol' truism "every ounce counts" yer confined to less materials, thus more basic geometric shapes - like the good ol' triangle... That's a topic for a different section tho
To summarize - this thread is NOT a cruisers vs mountain bike fight, that's pointless and won't fly
intercede if needful) Back to the OP's question, which my suggestion would be to just weld in a few gussets if yer worried - or better yet, jus' get a higher quality bike no matter the frame style.