The R-7 was never a practical weapon and the Soviet Union would primarily rely on strategic bombers for their war plans for many years after that. Even during the Cuban Missile Crisis, the Soviets only had a few dozen ICBMs (which is why IRBMs being in Cuba, right next to America, was relevant and desirable for them), the bulk of the threat through the 1970s and into the 80s was bombers.
Canada never needed a fighter-bomber, and it is a matter of historical record that they did continue to need aircraft for the interception of Soviet bombers, plainly evidenced by Canada purchasing aircraft specifically for that purpose. Those aircraft were not built to be dedicated interceptors but that role is all Canada ever needed and used them for. They would have been better served by aircraft built for it.
The Soviet ICBM threat was not relevant until many years after the Arrow was canceled; if you knew that before I told you, then I wonder why you even brought it up? Because Diefenbaker did? Diefenbaker was an idiot. Taking his excuses at face value is extremely dumb.
BTW if you actually wargame it out with realistic scenarios (accounting for the existence of AWACS and the probability of interceptors being in the air already), cold war US Navy interceptors easily get kills on Bears before they get their shots off, and can intercept the ASMs themselves as well. What makes or breaks it is the number of missiles brought to the engagement by both sides. It is by absolutely no means a sure thing for the Soviet bombers, I have no idea where you got such a stupid idea from.
The R-7 was never a practical weapon but interceptors were never a practical counter to bombers, and that got even more true when nuclear weapons stopped being gravity bombs.
Notably, the US Navy ran into the exact same problem. The USSR invented a bomber launched cruise missile to take out carriers, and there was no interceptor capable of getting into the air and into engagement range after the bomber showed up on radar but before it was able to launch it's missiles.
They spent like 15-20 years going down different paths (starting with https://en.wikipedia.org/wiki/Douglas_F6D_Missileer) that were sometimes silly and usually a bad project. The eventual and actual solution was: Better airborne early warning doctrine and systems, better picket and CAP doctrine, better ISR, and most importantly of all; enough advancement on missile systems that they actually COULD hit the bomber you selected reliably and reach out and touch them at fifty miles, and now basically any fast multirole fighter you have is a "bomber interceptor".
The dedicated interceptor plane was a interim role that was dying when the Arrow was cancelled. Average fighter plane speed was increasing enough to make dedicated hypersonic interceptors a poor plan, and missiles fly at Mach 4-8 anyway.
Bomber speeds also did not go as high as predicted. The bulk of the threat was bombers, but the investment into the kind of super duper fast bombers that the XB-70 represented ended for about the same reason: Missiles and radar got good. The US did build the B-1b, but that doesn't break Mach 2. The TU-160M does hit Mach 2.3, but that's as fast as we went.
Bombers moved on to low level flying to attempt to survive, which limited speeds anyway, so that's another strike against dedicated "Put a wing on a huge engine" interceptors.