It's important to have an answer to that question, rather than to assume that being offline when your home internet is down is inherently a problem. You can safely estimate using "X nines will cost X digits per month":
1 nines, 36.5 days/year downtime is $#/month. (openwifi tier)
2 nines, 3.65 days/year downtime is $##/month. (residential tier)
3 nines, 9 hours/year downtime; are $###/month. (commercial tier, datacenter tier)
4 nines, 1 hour/year downtime, is $####/month. (datacenter tier)
5 nines, 5 minutes/year downtime, is $#####/month. (carrier tier)
Speaking from experience, it's both important to decide which 'nines' you require before you invest in making things more resilient — and it's important to be able to say things to yourself like, for example, "I don't care if it's down 4 days per year, so I won't spend more than $##/month on hosting it".
https://en.wikipedia.org/wiki/Bell_System
https://en.wikipedia.org/wiki/Xfinity
https://en.wikipedia.org/wiki/Verizon
etc.
The way we sliced up space for utilities (lots of legal shared monopolies/guided capitalism) and their desire to build the last mile in their area leads to many different prices and products within a walkable distance. Before that 500/30 service showed up the best we had was unreliable 200/15 from another provider.
Regular 1Gb/1Gb is around 40CHF/40EUR often.
https://www.init7.net/en/internet/fiber7/
they explicitly (used to?) market that they don't throttle but simply connect at line-speed "because we can"
Though the small cost is probably overshadowed by the large infra costs at home. So now you need a 25Gbp/s router, together with the rest of topology like qsfp+ switches, and then actually computers with >= 25Gb/s nics to make use of it. And then all the appropriate cooling for it. It’s starting to sound a lot like a home data center :P
"in the worst case" being the key point, and frankly, 20 Mbps doesn't actually sound too bad as the theoretical minimum.
In practice you're unlikely to hit situations where this is a problem even if everyone was hosting their blog/homelab/SaaS/etc.
This is only a problem (and your ISP will end up giving you hell for it) if you're hosting a media service and are maxing out the uplink 24/7. For most services (even actual SaaS) it's unlikely to be the case.
It wouldn’t work for high bandwidth hosting, but works fine for residential and for 4 simultaneous video calls mostly all-day long during COVID.
Obviously it'd still be a bad idea to run a high traffic server on a residential connection, but as long as you're not streaming 4K video 24/7 or something you'll probably be OK.
But: Raspis these days work 100% fine with SSD's, and while a small SSD is not yet as cheap as an SD card, it's not far off.
I have entirely stopped using SD cards for my Raspis for quite a long while now.