If you can't succeed without first getting mass adoption, then you can't succeed. See the 'success' of Medium, and how it required losing everything they set out to do.
If Arbital has failed, Arbital has failed. Building neoTumblr and hoping to turn it into Arbital later won't make it fail any less, it will just produce neoTumblr.
Arbital has vague positive affect from being an attempt to solve a big problem in a potentially really impactful way.
Yet Another Blogging Platform, without the special features envisioned originally, is not solving a big problem (or actually any problem), and has a maximum plausible impact of "makes you a bunch of money and you donate that somewhere". Re-using the name is a self-serving attempt to redirect the positive affect from the ambitious, failed, altruistic project to the mundane, new, purely-capitalistic project.
Why aren't you just admitting defeat and going on to build something different?
I don't think I've ever used a text that didn't. "We have" is "we have as a theorem/premise". In most cases this is an unimportant distinction to make, so you could be forgiven for not noticing, if no one ever mentioned why they were using a weird syntactic construction like that rather than plain English.
And yes, rereading the argument that does seem to be where it falls down. Though tbh, you should probably have checked your own assumptions before assuming that the question was wrong as stated.