It's a snow day, so I thought that I'd have go at Y-Wing lore. I've never much liked canon for the ship, so I'm going to rewrite it, making the Y-Wing analogous to the Mosquito fighter-bomber of WW2. The upgradability of the Y-Wing is analogous to the continuous upgrades that Merlin engine underwent during WW2, which sustained the Spitfire as a viable fighter through the entire war.
Where the Star Wars universe differs from WW2 is in its mature starship technology. Decades won't be making a huge difference between craft. In our world, aircraft such as the A-10, the B-52, the F-15, and the MiG-21 have taught us that as long as an air frame is useful, it sticks around. Best, it seems, isn't always best. Even more interesting, countries of limited means have begun using jet trainers as cheap fighters. Some have even revived the piston engine for close air support. These vehicles may not be the best, but they do the job.
The Y-Wing was designed and developed by Koensayr Manufacturing in the tense years before the Clone Wars. Aimed at smaller planetary markets, the vehicle was intended as an escort fighter, easy to fly and cheap to maintain. Despite this goal, the eventual production model proved overweight and underpowered. According to those early pilots, "it put the dog in dogfighter."
The Y-Wing would have gone down in history as a market failure if not for the Clone Wars. The sudden outbreak of war put all available fighters to work while manufacturers were slammed with a titanic backlog of orders. Given extensive delays for any replacements, militaries modified their existing Y-Wing fighters by removing unnecessary weight and by over-powering their engines. With those field changes, the humble Y-Wing unexpectedly proved itself the best cheap starfighter in the galaxy.
Because these fighters were so easily modified, planets were able to retask these fighters into numerous roles, such as scouts, ground attack fighters, minesweepers, couriers, and torpedo boats. By the war's end, these fighters had become the predominant local starfighter in the Outer Rim. General Dodona said, "They did everything that we asked of them and more."
With the rise of the Empire, the need for region defensive craft diminished. Planets that voluntarily accepted Imperial Garrisons were required to scrap their local fighter groups, and so the Y-Wing quietly disappeared from arsenals and resale lots. Planets that resisted the Empire quickly saw their fighters destroyed.
During the rise of the Rebellion, salvaging surplus Y-Wing became mission #1. Their ease of repair and ease of modification was exactly what the Rebellion needed. Aggressively recruiting veteran Y-Wing ground crews, General Dodona's personnel rebuilt the vehicle far beyond its original specifications, producing a strike fighter capable of matching the new Imperial TIEs. The resulting Y-Wings proved so capable that they led the Rebellion's desperate attack on the Death Star, completing a suicidal trench run to fire on a thermal exhaust port. Later in the war, through the dogged innovations of the ground crews, the Y-Wing continued proving its worth, maintaining its place in the Rebel arsenal despite the introduction of newer craft.
For most of the war, Y-Wings performed the bread and butter operations that sustained the Alliance, freeing up the more expensive craft (such as X-Wings, A-Wings, and B-Wings) for higher profile missions. According to records, Y-Wings flew 70% of all sorties. After the war, Y-Wing pilots often boasted, "The X-Wings got all the glory, but the Y-Wings did all the work."
Overall, I liked how this worked, making the Y-Wing the scrappy underdog of starfighters. It exemplified what makes the Rebellion great, by taking the ordinary and making it extraordinary. Rather than giving it the story of, "it's old and not that good anymore," the story becomes, "somebody's got to do the work." The Rebellion's use of the Y-Wing represents the exact opposite of the Empire's approach towards military technology.