Spacewalk danger, outside sub-systems; no ISS/shuttle refuge?

We're told, that this spacewalk, is to replace a "failed power component".
So, given the risks of spacewalks, why are any sub-system modules placed
outside? Offhand, it would seem best to maintain and change replaceable
units, from within. If you have circuit boards, you'd build a module so
access would be on the inside of the craft.
The only reason I can see, is fire danger in an oxygen rich field. While
testing circuit boards, I've seen ICs, caps, or resistors, fail and burst
into small flames. If such a card had been outside a spacecraft, there
would have been no flame. I wonder how this is engineered. Or could be.
Build each card-cage/rack into a air-tight/space box. The inner access door
would have an air-tight gasket. There would be a very small pin-hole on the
outer wall, with a slide cover to be used during access. Normally,
after access, the inner air-tight door would be locked. Thus, any air would
be slowly drawn outside. The card-cage would usually be in a vacuum.
For access, the outer cover would slide over the pin-hole, and inner door
opened, to let in some air, so the door could be opened. Any wires to and
from this box, would have to have vacuum proof seals, (or vacuum proof
bulkhead connectors). Normally, the slide cover over the pin-hole, would be
powered, and controlled. If power failed, the inside door air access covers
could be opened by hand.
Once the box had been flooded with air, the inner door could be opened,
and the outside slide cover also moved by hand, to close over the small pin-
hole letting out air.
But then, during the last, (deadly), shuttle flight, at one point I
felt, something was not right. And maybe they should seek out the ISS, and
via airlock, or spacesuit, find refuge. Even if Nasa found that shuttle
could never land, could the ISS have helped? Say, their optics viewed
a shuttle with no left wing? Was there no way, that shuttle could
not find/join the ISS, and transfer the crew? Don't both have airlocks?
Sent up enough shuttle spacesuits? Maybe with some spare oxygen
and food, to carry over to the ISS during an emergency?
Didn't NASA have any hint, something was wrong? If so, they could have
sent out a spacewalk. With the fatal flaw revealed. That the shuttle could
never return to earth. If the crew couldn't survive via the ISS, they were
doomed. If so, why not? Fuel, orbit, time, air, speed, or what? Interlocks
or airlocks? Was it a moot point? If launch video showed insulation damage,
why couldn't there have been a spacewalk to check on this? Send out an
expert, to say either, "Looks OK", or "There are some large holes in the
shuttle's insulation". "Looks OK"; and there could be a chance for landing.
If not, seems the crew could only have looked towards the ISS, for a chance
at life. Frozen O-Rings, or damaged shuttle insulation; I see NASA doing a
gamble on both. They could have paid more attention to warnings about
a shuttle launch at zero temps. And they had info about shuttle insulation
damage. Any hint, why not to explore? Take a few more orbits, and send
out someone to check under the hood? I don't care if that shuttle cost
a billion or more. Just do the most, to keep the crew alive. There were
hints, this last shuttle had problems and was doomed. What if that crew
had other 'options'; to survive in 'space'?
We've got an International Space Station in orbit. Seems any nation that
sends humans into space, should be able to use the ISS, as a 'safety raft'.