Besides many of the excellent points raised previously...,.Mitchell cornered the market in the 1950's with the Garcia distributorship rights deal introducing them into the US market ...something so highly sought that Bache Brown tried to get those rights himself. This success allowed Mitchell to introduce many other products like the 302, 306 and 308 that nailed even more market share. They also had the 304's and eventually 314's that being cheaper could introduce even more users to their products. About the only initial European competition they had was Alcedo, and Zangi...and Alcedo was much more expensive. Zangi and later on DAM and ABU gave them a run for their money.
Then there were the 400's...which eventually provided a version for every reel Michell made, with the exception of the 304/314's, givng people who wanted higher gear ratios what they desired...and eventually line rollers instead of simple guides.
The 302 was probablely a challenge, but every other reel they had could be field stripped like a military rifle...making durable designs even more longlasting with the the ease of maintenance possible. All the 300 required was not losing the shims...other reels were complete no brainers.
Eventually, all their reels had pop off spools allowing one to maintain a set drag, when the 304 was upgraded to the 314 configuration. That pop off spool concept was copied by very respectable manufactures like ABU and Dam Quick.
Personally, I wish that they would have offered a 308/306 type design reel in 300 size, though I get around that by using a 314.
About the only bad thing about Mitchells was that their finish wasn't as durable as other maker's products....something I believe began when they seemed to abandon using primer...but that is a cosmetic issue.
Mitchell was also very tuned into line lay consideration...having three different designs, levelwind and crosswind for the 300's and planamatic and crosswind for their other products, to address such issues.
Eventually, they were surpassed by ABU and DAM with what many consider a much better gearing design....and ABU's began to penetrate the American market as well. Other issues also effected Mitchell negatively...like attempting to keep their products at a price point which required sacrificing quality.