If El Cap loses support, where to update to?

[baas]

So here's the situation. I've got a 13" MBP early 2011 (8gb ram, ssd) running (low) Sierra without any problems. I also manage a 2010 mac mini (4gb ram, SSD) and 2014 mac mini (8gb ram, hdd) both running El Capitan for my family. They're mainly being used for basic tasks like browsing, mail and office.
All machines are running well, but I assume that with the release of Mojave, El cap won't receive security updates anymore. That rise a few questions and choices.

- How insecure is it to leave the mac mini's on El Cap? In my opinion this is one of the more 'lightweight' snow leopard like release of the past few years. I could switch the browsers the firefox for example so those will stay up-to-date. On the other hand, my family members aren't really IT pro's so giving them maximum security to prevent phishing or malware might be more important then performance alone.
- I can leave my MBP on sierra as it will still receive security updates for another year or update to HS since it won't run Mojave. Another option is to patch it with dosdude's patch and install Mojave, but I wonder how it's performance will be.
- The 2010 mac mini and my MBP won't run Mojave but I'm hesitant to update them to High Sierra since a lot of people seem to have had trouble with it (especially in the beginning). I could update it to low Sierra but that will give the same dilemma next year.
- the 2014 mini I can upgrade to Mojave when it comes out, but I'm wondering about it's performance since it doesn't have an ssd. Ideally I would upgrade that first.

I hope to hear any thoughts!

 

[oldhifi]

I plan to keep EL Cap on my 17 in MBP as long as I can, then I will go back to Linux..

 

[Argyboy]

You'll still get third-party application updates (chrome, dropbox etc) for another couple of years at least, so I'll keep El Cap going on my Mac Pro 2,1 for as long as I can, and then probably use Windows or Linux if the machine is still kicking by then!

 

[Mr. Retrofire]

...
- How insecure is it to leave the mac mini's on El Cap? In my opinion this is one of the more 'lightweight' snow leopard like release of the past few years. I could switch the browsers the firefox for example so those will stay up-to-date.

The OS itself is not very vulnerable, if we exclude attacks which require physical access to the machine. Up-to-date browsers are important for your security. I recommend Chrome, because it is fast, Google fixes security problems faster than Apple and Chrome has many phishing / malware protection settings. Firefox is also a good choice.

- the 2014 mini I can upgrade to Mojave when it comes out, but I'm wondering about it's performance since it doesn't have an ssd. Ideally I would upgrade that first.

I would upgrade the 2014 with a 860 EVO for High Sierra, not Mojave. This SSD is faster, more energy efficient and cheaper than the 850 EVO and with the same TBW (Tera-Bytes-Written) rating as the 850 PRO.