They succeeded where others failed because they are fun to drive.
There were no fun EVs prior to the Roadster. And the Roadster was a small production precursor to Model S, which transitioned from boutique-ish to mass production, and which led to the Model 3, and then the Model Y, the number one selling car in the world.
Until the past five years or so almost every other EV was still an oddball whose design language seemed aimed at screaming "EV" over anything else. Look at the i3 or the Nissan Leaf. Until Cybertruck all Teslas retained a fairly conventional appearance.
No they succeeded because of carbon credits. To this day, 42% of Tesla's revenue is just carbon credits.
That means that everyone who is buying a gas guzzler is helping fund Tesla. Tesla was one of the first to have an EV-only lineup which means they could sell 100% of their carbon credits. Every company that wants to sell a gas guzzler has to pay Tesla for carbon credits in order to do so
The roadster was a conversion of a Lotus Elise, they were still trying to figure out powertrain and battery back then. Still, reusing an existing chassis was a good strategy till they had more experience.
Tesla succeeded because it focused on the luxury market first by building the Roadster (and then later the model S). It succeeded because they showed that electric cars can be cool and not associated with lame-looking and performing Priuses and Leafs. It succeeded because people bought billions of laptops and cell phones powered by lithium ion batteries, which finally produced the economies of scale and had the right properties that made electric vehicles viable in the first place. Subsidies came later, and Tesla would have done fine without subsidies. Its the trad automakers that are attempting electrics that need the subsidies.
It succeeded because it was the only company to only sell EVs. That means everyone who wants to buy a gas guzzler is funding Tesla. In the past decade, over 40% of all of Tesla's revenue is carbon credits
The fact that an EV failed without that kind of publicly funded support is only evidence that furthers the point GP was making