I do recall reading that these will be the last generation that will be supported with OS upgrades, so the next major OS won't work well with this generation.
Even if this Mojave (10.14) update is the last OS for your 2014 MBP you'll have 3 years of updates if Apple follows past protocol. You should be fine until fall of 2021 at a minimum.
We won't know until next summer for sure, but it seems like there is a good chance you'll even get another year out of your 2014 to the fall of 2022. It was the 2012's that dropped off this year and now only have two years of updates remaining. 2013's are definitely on the bubble for their last OS next year. Hopefully not 2014's.
I'm very familiar with all this as my beloved 17" Macbook Pros are coming to the end of their life. Even worse Apple doesn't make them anymore.
My 2009 running El Capitan (10.11) likely received its last update recently and will be a security risk going forward. My 2011 running High Serra (10.13) isn't upgradeable to Mojave, so I've got two years of updates coming before it's unsupported.
Anyone running El Capitan or older should be looking to buy a new machine, or be a super geek willing to jump through hoops to implement other options to keep yourself secure. There are too many bad guys out there to be running an OS that doesn't receive security updates!
Yes, battery life is one of the top considerations, and it is not just adding an hour or so but very well doubling or tripling the life. Right now I am getting 3-4 hours and the new ones will get 10-12 hours.
I doubt you'll see that much difference with the same usage. How many cycles do you have on your battery? With a good battery your current Pro will give you 90% of the battery life of a new Pro when used the same way.
Your current Macbook Pro is rated 9 hours:
https://everymac.com/systems/apple/...-i5-2.8-13-mid-2014-retina-display-specs.html
The new Macbook Pro is 10 rated hours
https://www.apple.com/macbook-pro/specs/
I do think you have a good case to switch to an Air for better battery life. An Air will give you something like 133% of the battery life of your current Pro. I'd just have a hard time spending that much on an Air to likely take a step back in performance from your current Pro other than battery life.
The new Macbook Air is rated 12 hours:
https://www.apple.com/macbook-air/specs/
Note the old Macbook Airs are also rated at 12 hours:
https://everymac.com/systems/apple/macbook-air/specs/macbook-air-core-i5-1.8-13-2017-specs.html
I'd think about picking up an older, 2015ish, used Air for $400-500. You won't notice hauling them both in the car and combined you'll get more than double your current battery life. Just an idea.
Your 2014 MacBook Pro is too nice to let go for $400, IMO. I'd keep that around for cheap redundancy whatever you do! I'll buy it for that in a heartbeat, as I suspect others here would as well.