Sewers tend to have a design lifetime of 50 years, although things usually last longer. Expect to see a lot of sewer replacement projects in suburbs in the coming years. The good news is most of that can be done in pieces. The bad news is some municipalies don't have a good inventory of sewage infrastructure or a good plan to catalog it, so they don't do much proactive replacement, and reactive replacement of sewage infrastructure usually involves spills or backflows and is pretty gross.

It's a lot easier to find new money to build infrastructure to new residents than to find new money to rebuild aging infrastructure, of course. But, it's also easy to scale up or down infrastrucure maintenance to fit your budget for the most part. If you have a small budget, you clear problem pipes regularly, rather than replace them; if you have a large budget, you identify pipes that are becoming problematic before they cause problems and replace them, and build in redundancies when possible; somewhere in the middle, you replace the worst pipes when you can afford to, and clear the others. Few systems will outright fail and the city needs to be abandoned (but there was a story of one in the suburbs of St Louis in recent months... Although, again that was a long decline, not a sudden issue.)