Morning all!
Well, I've done what they say to do.... I read the thread, I counted to ten, I walked away from the computer and I even had a coffee but on my return all I can say is still..OMG!!!
I like the 'sky God in a spamcan' comment - well done!
Here's my 2p...
The mark one human eyeball is and always has been your only form of defence, if your aircraft doesn't have a back window - look out more!
If you strap into one of the queen's speedy little darts with a big helmet on and God knows what else restricting your movements and holding you down, that does not open up the excuse "I cant see, sir!" The answer will be - Look out more!
I seem to regularly be saying to people these days that we don't sit there like Captain Kirk with his 'sensors at maximum' we have to look out of the window whether at 100mph or 1000mph. The non flyers in the audience often look askance but this thread seems to be started by a qualified pilot who doesn't understand that either.
There is no excuse for not seeing an aircraft in the sky wherever it is, there is no wriggle room in this sport/pastime/industry, call it what you will, for this kind of slack behaviour these kind of errors - KILL PEOPLE, usually you and often someone else as well.
Of course, as human beings we are all capable of failure and I like everyone else has at one time or another suddenly noticed another a/c but to start from the viewpoint that it was competely the other guy's fault shows a dangerous attitude and probably a lack of training in this area. Consider for example the type and frequensy of scan you use, are you focussing on the GPS? or anything else in or out of the cockpit, ideally, your head should never stop moving.
Also, at the risk of 'going on a bit'

you need to figure out what assistance to expect for the different types of ATC service. In your post you said that perhaps you should have had an RIS. Unfortunately, this again shows that you are trying to absent responsibility and control from the cockpit - where it belongs - to an outside agency who have neither authority nor control of you in the open FIR.
On the plus side no one was injured and if the responses on here, which happily all seem to be in accord, fill in some gaps for you and maybe the other guy too if he also visits here, then thats good. Remember though, experience will only serve to instruct if you let it, arguing that it was somebody elses fault, although the 'modern way' will not help you or the rest of us in the sky.
Apologies for the length of the response, must be a bit cranky this morning, but I genuinely feel there are few more serious and basic issues than lookout, and so little time is devoted to it now it is quite possible to imagine that it doesn't matter!
Rant off.
Regards to all
Happy and Safe Flying
Xraf