When you feel the urge to buy pull out that phone & look at your beauties 
Think of what getting that new chick would take away from them. The space, your time & attention, sharing treats etc. You can think of it as another way to spoil your current flock by letting someone else take on that new chick.
After you have bought feed store chicks numerous ti. Re s and had them die , even after you have fed them 5 months, you get cured. Stores dont ever spend the money to have them vaccinated. Buy from Hatchery and have them vaccinated.
There is a fairly high loss ratio in chickens. You need to figure it in. Just the way it is.Half my flock are technically TSC birds only they were purchased online and shipped directly to me from the hatchery that supplies their store.
The hatchery is the first link in the chain of hurdles you could face since they hatch in such bulk birds with problems are very rarely caught.
Once they hit the store they face less than ideal conditions when they're already stretched thin even if they're perfectly healthy.
I dld get birds with issues, out of ten one has comically short legs with one foot noticeably larger than the other an one that spins in circles.
Since the chicks were properly cared for and skipped the store experience entirely I didn't lose a single one and they're now coming into lay.
Also 3.5 to 5 years absolutely can fall into the normal lifespan of a laying hen.
You aren't guaranteed a set number of years, just like us when time is up they're gone no matter where they came from.
