e.g. savings accounts with 1% interest in the EU which are government insured up to 100k. Plus mortgage debt tends to have fiscal benefits in most countries, too. That's a better idea than paying off a 0% mortgage. Especially past a certain point where there's plenty of equity in the home.
There's generally virtually no financial reason to pay off a 0% debt apart from some edge cases here and there. People do it for peace of mind, which is of course fine, but it only works because of financial illiteracy as it's not the most financially optimal decision to make. (even not the least risky).
Meanwhile, getting 1% return on a 30 year mortgage puts you in a much safer, financially cushier position, than having had 0% returns for that period in time. Would you rather have 40k saved + 10k interest saved up in savings saved up in a government-insured savings account when you lose your job, or would you rather have a 40k lower mortgage debt and slightly lower payments or a lower time-to-payoff? Particularly given that mortgage payments can be furloughed and negotiated in times of payment difficulty. The answer is very clear. Choosing 0% returns over 1% returns in this case is the riskier choice.