Canon isn't going to give you L quality and performance and not charge a premium for it.
What is L quality and performance?
It's a term I've often heard, but I don't know what specific criteria it refers to.
It's not weather sealing because I own and have owned L lenses with and without weather sealing.
It's not AF performance, because the AF on my old TS-E 24mm f3.5L was lousy to the point of being non-exsistent.
It certainly isn't MTF figures on their own, becuase there are many non-L lenses that match or exceed L lenses in some resolution regards, such as the fast short telephoto primes.
I'm knowingly being pedantic here. I know L is a marketing term that in actual fact means you have a red ring round your lens and thats about it, but I always have to question those blithely using the term 'L quality'. If it's quantifiable then go on, quanitfy it.
What is L quality.
I don't think anybody is realistically expecting the 55-250 STM to have Canons best build, weather sealing, or the fastest AF, or smoothest bokeh etc etc. But here's the rub... It's canon doing in some ways what they do best, what got me using canon EOS in the first place back in the early 90's, what keeps canon top of the market..
Right kit for the target market at the right price.
Anybody who jumps on giving budgets users half decent kit (with things like FTM and non-rotating filter threads wowsers!) because it isn't 'L quality' just looks like a snobby tit.
This lens isn't for the most serious of users, or for the phallic extension rich dads at schools sports day. But it's the right lens for a lot of folk, and thats why canon will sell shedloads, and stay in business.