And you are a brave and wise man by not doing so. The construction is as follows:

- inside the hinge part of the main frame there are bronze bearings.
- through the hinge part goes a steel spindle, that sits on those bearings. The spindle is slightly longer than the hinge part of the main frame
- the rear frame is sitting on this steel spindle, so the load goes onto that spindle, not onto the bolts.
- between the rear frame and the main frame there sit slim nylon shims on each side
- the bolts are just there to keep the rear frame on the spindle, very much like the nuts on a bike wheel's axle. They press from the side and do not get any vertical load. Those bolts are conical and they are glued in.

Over time the bronze bushings wear which leads to side play in the rear frame. In this case the bushings have to be exchanged. This is an ordinary, well described maintenance job, the parts are cheap and so is the work. It is however nothing for the ordinary hobbyist or fainted hearted as the bolts have either to be drilled out from it's heads or cut in the gap between main frame and rear frame as they are glued in. The bronze bushings have to be removed, the whole thing has to be reamed with an exactly fitting reamer, then the new bushings come into place along with a new spindle and the whole thing goes back together. If the job is not done properly the new bushings will wear quickly or will not stick, the play will not be gone, the whole thing will not fold properly or you'll loose the bolts. Thus it is a good idea to give this work to an experienced workshop that knows what he is doing because attention to detail is relevant during this operation. Many Brompton dealers do not know how to do it properly and delegate this work to someone who knows (which is a good idea) while others in the same boat deliver bad work. Ignoring massive play for too long can lead to a state of unrepairability due to overwear and a damaged main frame.

When a bolt sheers this is not only very, very rare but due to the fact that the glue that holds it in place was either not sufficient, had not enough time to harden or was dispersed. The latter happens if the owner of the Brompton has i.e. the idea to lubricate the thing with oil. Longterm it will loose the bolts and there you go. If you leave it alone as you are supposed to nothing will happen. When the bolt comes loose the rear frame can move sideways on the spindle and give load to the bolts that they are not designed and intended to handle and so it sheers. This is however a process and it needs a bit of an ignorant rider not to recognize while riding that something is seriously wrong when a bolt has started loosening. So in most cases nothing sheers, you glue the bolt back in and tighten it, job done. And even that is a more than rare event.

Same goes with the bushings btw. - if you lubricate those (and possibly overlubricate) they will come loose and the trouble starts.

My question was if there was not something longer lasting and more sophisticated than the bronze bushings but I was told by an material expert for those kind of things that this way of handling the thing is still state of the art and perfectly fine for the application.

The people who judge and claim that the whole construction would bad because the bolts are so tiny just missed to understand how the whole construction works. If they had they would know that the bold does not carry transverse load and is perfectly fine for the purpose that it is intended to deliver. And therefore they would judge differently and not claim wrong things. But judging ignorantly on things w/o even trying to understand them first seems to be a plague of the modern age.