I have a few moderately successful Cubecart stores, I've been a customer for many years now, I think I started in version 2, I've flirted with other carts over the last decade, including a disastrous stint with Oscommerce but always seem to revert to the known. I've got an Opencart store on the go as well but I hate it and Magento is way too cumbersome (and I can't be bothered with all the phone calls of them trying to up-sell me), the rest (Shopify, Bigcommerce etc) are all on their systems, which I'm a bit too paranoid and or cheap to sign up to.
I kinda don't know what your objective is, your reasoning is a bit flawed, I'm a business owner too so I get the caution, but I don’t understand how this is an issue here?
For example, my latest store and new business venture I also had a ~£0 budget, there was a bunch of reasons why I didn't want to just spin up another cubecart. However, my final cost to launch was ~£150 there is no competitor cart you can dust off and run for 0 money and provide the same level of use. Your main gripe, the All in One shipping module, is free, there are a bunch of other free extensions that you need to install to have an operating cart (PayPal). I don't see how that's an issue, some people will only use Sage not Paypal or like me only ship to the UK for free postage, so if they don't need it, it' s not installed, software is lighter weight. Also the creator of All-in-one hasn't been around in a number of years, so the idea of giving the keys over to them, is impractical as well as a bit mad.
You do highlight an interesting point though, there is almost zero reason that I can see that the All in one shipping module isn't standard at this point. It was always a bit of a glaring hole in the software that has existed since at least version 3. I personally have always needed that mod, but I assumed they never baked it into the software because didn't want to step on the toes of a 3rd party extension creator (who even at the time where few and far between)?
There are some minor issues that I have with cubecart, that have been around for years that seem to just go on “unfixed”;
The search function is just bad
The look of the default skin is a bit dated
The management of orders in bulk is non-existent (Bsmither custom made me an export or I'd be screwed)
The 3rd party marketplace fairly small and it’s all on the back of like 3 guys and seems to be getting smaller over time.
There seems like Quality of life upgrades that could be made that aren't because, maybe I'm the only one seeking them (which I admit could be a possibility), there isn't the development time for them or that they are waiting for the 3rd party extension to be made if the demand is high enough from a community that hasn't really existed or recovered from the switch from opensource to paid and back again *(In my opinion)*.
The main strength of Cubecart is that it's very modular and it's easily customisable, the main detraction seems to be I'm not skilled enough nor have the time to do it myself and to hire someone to do it seems cost prohibitive.
There are 7 paid for themes on the marketplace right now, some of them haven't been maintained in years and look straight out of the 90's, one's demo page links to a Shopify site, one is an easier way to customise the Foundation skin, which is neat but not what I’m looking for.
I can go chose from 600 OpenCart themes and sure most of those are themeforest hatchet jobs straight from coding hell but hey they look pretty.
So I thought to myself, “Hey, you pay money for a theme anyway, what could be the cost of just hiring some guy to build exactly what you want” now admittedly I also wanted to hire locally so mileage my vary, so I emailed 8 web designers. Two actually responded, both wanted to migrate to Magento to install some random stock skin for a few thousand. Gave up with the self promise that in 2020 I'll finally make a go at that skin I started in 2018 :D.
So in closing, my own gripes though detailed and long outstanding are fairly minor in the face of the utility, low cost and ease of use that Cubecart provides. I think you are right to have your complaints, I kinda half share them but I don't agree that cost to launch is a good argument to base it on.