End of life for Airport express, AirPort Extreme and time capsule. When should I stop using them?

When you replace them, of course, will be totally up to you. At some point they will start failing and you basically have two choices going forward: 1) Replace them as they fail, or 2) Replace them all at once with another networking hardware solution.

The other thing you bring up is network capability and security. Chances of Apple releasing any firmware updates for any of the AirPort base stations is pretty slim and I certainly wouldn't count on them. These devices were designed more than 8 years ago, an eon in networking hardware, and have been eclipsed by just about everything available today.

FWIW, I opted for the first approach and, since my home was pre-wired for Ethernet, I went with Ubiquiti's UniFi line. I started by replacing my AirPort Extreme base stations with UniFi wireless access points and managed switches until the only Apple base station I still continue to use today is a single Time Capsule ... now deprecated to a simple file storage device. The UniFi line is considered a serious home/business-grade level and some of the original founders came from Apple ... so you can see some similarities. However, unless you are a bit of a networking ninja, you may find them having to require a bit of a learning curve.

There are many other options out there that do not require an Ethernet backbone if you want to go all wireless.

if you mean in terms of security from online exploits then I would not worry too much, the people who look for security holes in equipment usually go for equipment with a rather large user demographic, apple never been a huge player in the router market.

i'm replacing my 2 airport extremes with a linksys mesh but will likly keep the 2 express to use for airplay