My understanding is the onion service was launched to provide access for people in places where Twitter is blocked. How is this "irrelevant to the business"?
I'm not someone who's ever used the onion service, but as someone who accesses the site regularly over VPN, I can testify that it's a huge PITA. Tons of very aggressive captcha-type challenges (various tasks, never simple text recognition, but not Google ReCaptcha), often 5, 7, or 10 to be allowed to proceed.
If it's a safety mechanism that people relied on, leaving it to rot and getting people in the habit of clicking through on an expired or invalid certificate warning is irresponsible.
A degraded service can be worse than an offline one.
