The 70-300 IS USM is a cheap lens and suited to its price point - below that of Tamron's 70-300 Vi DC.
In which market? In the US, the Tammy is $ 450 and the Canon $ 650.
If you look closely at the 24-105 STM MTF graphs and compare them with the 24-105L graphs then it is possible the STM bests the L for pure IQ and is cheaper too! That sets the stage...
Yeah, but you are paying a price premium for the build quality, the L prestige, the quickness of USM and ergonomics. With a standard build quality, no L, micro USM and mediocre ergonomics (rotating front filter, no manual AF override), it will be impossible to justify keeping the 70-300 at its price point with a 70-300 STM that performs better. Having said that, I think it will be hard for Canon to build a 70-300 STM that performs any worse, and the price point of the lens isn't justified even now, to begin with.
One thing puzzles me...
Why stocks mediocre 75-300mm never end?
Does Canon will have to sell them for $ 50 to get rid of them.
I wondered about that, too.
Except people buy them quite a bit, paying about $ 200. I know three guys who did, and I was shocked that they didn't choose one of the 55-250s instead. The main reason is ignorance about other options, strong marketing by the sales people (I have heard it myself) and low price point.