Quite a lot of tech companies are using .io nowadays. If .io reliability is an issue what is the next best alternative for tech domain names?
Keep a .io domain or whatever you want for marketing purposes if it's absolutely necessary, just realise your splash page is at risk of failure some times. Whether it's worth the trade-off is likely up to the individual company.
.com is the best not-alternative
Note that some of these newer TLDs can be more expensive (up to $60/year).
.io and .com are short and easy to remember.
Then this answer is not helpful, this whole article and the parent's question were about reliability.
A .com WILL also be expensive, because all the good names and most of the crap ones are either used for other companies or, and that's worse, for squatters who hope to extract five-digits prices.
Disclaimer: I work for Donuts, owner of many new TLDs.