Lenses with large entry pupils use rear drop-in filters with 52mm threads.
A 135/1.4 would roughly weigh 2-2.5kg like a 400/4 DO, 200/2.0 or 300/2.8 as they all have similarly sized entry pupils. It would sell for roughly $6,000-9,000.
Sony's 135/1.8 is designed for a 77mm thread filter. One with IS may require a 82mm thread.
A rather old lens (1993) the 400/5.6 is ripe for a IS version.
The main reason Canon does anything is in reaction to what the competition does.
A 135 update is very possible because of Sony.
Nikon have updated these lenses of late. I expect Canon to update theirs to catch up. This list in not exhaustive.
85/2.8 tilt shift
45/2.8 tilt shift
24-70/2.8 with IS is very possible seeming Tamron has one with VC. It may take Nikon to nudge them along though.
A new 135mm f/2L IS would be a dream all right - albeit sharpness can take a slight hit, maybe a 135mm f/1.4L? Probably they can find a way to let in more light in a longer lens but the lens could look like a longer 85 1.2L and cost too much.
135 f/1.4 - 135/1.4 = 96.4mm (105mm filter?)
135 f/1.8 - 135/1.8 = 75mm (77mm or 82mm filter)
135 f/2 - 135/2 = 67.5mm (72mm filter)
A 135mm f/1.4 would put it in the same class as the 200mm f/2 (100mm objective) and 300mm f/2.8 (107mm objective) both in terms of size and weight. A suggested 135mm f/1.8 is definitely more reasonable in terms of size, weight and price.
True, over the last three decades. Not quite as much over the last decade, and most of Canon's previous generation of lenses are within a decade old.The EF 24mm f/2.8, EF 28mm f/2.8, and TS-E 24mm f/3.5 are less than a decade old?
To put numbers behind it, Canon has made 159 different EF/EF-S lens models since 1987. Currently Canon USA lists 68 lenses (I take issue with 5, as they are no longer in production and no longer available new - 300mm f/2.8 IS I, 400mm f/2.8 IS I, 70-200mm f/2.8 IS I, 100-300mm f/4.5-5.6 USM, 15mm Fisheye). Out of the remaining 63 lenses here is how they break down by release date:
0-10 years - 37
10-15 years - 10
15-20 years - 9
20+ years - 7
So his statement is correct - 37/63 (59%) are 10 years old or less, which constitutes 'most'