When???

[Bduck]

What criteria you use to replace your electronics mainly a fish finder or combo unit? Does technology upgrade aid in replacement of electronics? Do you wait until glitches start overcoming performance or do you just keep using until it quits functioning? A lot of discussions involved is how some share how the units work helping with the purchase of, buy today and tomorrow its outdated or something new hits the market. Affordability? Next to my kicker constantly putting a number of hours running every trip, my combo unit sees same number of hours in use. When does it become enough to replace? 
I've had one unit used for many years to quit picking up fish no matter replacing skimmer. Another that went 6yrs which the screen went out altogether. Yet another combo unit approached 5yrs still in use for which I now use for its GPS but replaced with a larger 9" screen combo unit now at 4yrs old. I'm thinking of a new unit again which I have time that can go either way right now. I do like technology but pricing some units is hard to be swayed the expense, it could hit me to break down for a well deserve unit.
Where do you take a stand?