Currently the "solution" some are proposing to the Northern Ireland border issue is using technology that is either untested or doesn't exist. Extending article 50 would allow time to actually create/test something that works, then the UK could go back to EU and say "this is what we've got and we've proven it works", at which point, UK leaves EU and there's no hard border, everyone wins. Yes, businesses and individuals don't know what's going on because of the uncertainty, extending would give every company another 18 months at least of "carry on as normal", ending the uncertainty. Nothing really happened in terms of businesses pulling out of UK in the first 18 months after triggering article 50, it's only been recently. Extending would I think allow everyone to take a breath because we know a lot more now than we did then.
I agree that the EU is probably not going to renegotiate but that's not egotistical, that's them in a stronger bargaining position than us and using full advantage of it. If I was the other way around I'd hope we would do the same. Why should they allow us to have our cake and eat it? Because we're Great Britain, some world superpower? Cause we're not, we're a relative minnow in the world economy.