If the UK was in the Schengen agreement and there was no concern over security, I'm sure Eurostar would be run in exactly the same way as other international trains in Europe. Thanks to political opinion in the country there is not much chance of this happening.
In my view security is used as a reason to prevent a proper walk up service, but this would not be a problem if there will was there to overcome it.
The problem is that Eurostar has always been seen by the government of the UK as an aircraft on rails, and probably always will.
I think in those countries, a train ticket is usually tied to a specific train (I believe this is called "global pricing"; don't ask me why). This would explain a lot about their approach to rail passes.
I certainly remember InterRailing in France and Spain a few years ago (when it was still using the old zonal system), and always having to reserve a seat and pay a fee (and sometimes not being able to on a TGV because the quota had run out). I do sometimes wonder if I would have paid less by buying point-to-point tickets. That said, it allowed me to get one-way Eurostar passholder fares (at a time when Eurostar didn't have 'normal' cheap one-way fares and I didn't want to specify when I came back).
I haven't inter-railed for many years, not since the 1980's when it was all a lot different to today. From what I can gather, most of the long distance trains in France have low quotas for railpass holders, and high fees to book a seat if you can get hold of one.
I understand that Italy and Spain are a little better than France, but that in most cases the cheapest point to point fares are less than the equivalent Advance tickets. As there seems to be limited chances to just decide what to do on the day, you may as well plan your trip in advance and not bother with a pass at all!
Despite no Eastern European travel, I am quite glad I was able to inter-rail when I did!
I think you are right here, and I don't much like it either. It would be nice to have a traditional passenger train service through the Channel Tunnel (i.e. had mainly walk-up fares, and stopped regularly at all stations along the route between Lille and St Pancras). It would certainly change the way people think about travelling between the UK and France. The concept has been floated (and even discussed here, I think), and there is lots of spare capacity in the CT; but I doubt it'll happen any time soon. The principal reasons are (i) the over-strict safety-related rules about what types of train are allowed to run, (ii) the very high access charges for the CT, and (iii) passenger trains that use the CT not being allowed to carry domestic passengers on the UK side. Indeed I suspect that the ONLY types of cross-channel passenger train services that could ever be viable under the current regime are Eurostar-like long-distance airline-on-rails services to/from London.
So much needs to change in the UK before we can expect a proper international rail service like those between France and Switzerland, for example. I hope that it will happen one day, but I'm not holding my breath.