With regards to depths limitations, I feel that the OW, AOW and recreational 'Deep' limits are entirely suitable.
As an entry-level diver, being limited to 18m/60' allows you to enjoy a huge spectrum of dive locations, but still keeps you within a zone where an emergency OOA solo ascent (CESA) is entirely achieveable and relatively undemanding. Likewise, it keeps you in a zone where your nitrogen uptake is considerably slower and any loss of buoyancy leading to an uncontrolled rapid ascent holds far less potential for DCI complications.
As you extend your entry-level training, with the AOW course, it is logical that your buoyancy skills and ability to deal with problems/stress underwater will also increase. This means that lowering the depth limitation to 30m/100' is balanced against your increased capacity and skills.
Below the 30m/100' zone, we see a much increased impact from nitrogen narcosis, coupled with the 'double-whammy' of more rapid air consumption and very short no-deco limits. Thus, we are at a depth where our thought processes, concentration and focus are most diminished - but the impact of losing concentration can easily lead us into a error where we are both low on air and obligated to conduct a long, slow ascent with emergency decompression stop/s. Diving at the recreational depths of 30m/100' - 40m/130' demands more advanced training, skills and experience. Hence, the Deep Speciality course.
I view OW and AOW as primary and secondary entry-level courses (think of them as OW1 and OW2).
With regards to wreck and cavern diving, the agencies all have strict definitions and limitations on what diving activities can be carried out in these conditions by recreational divers. I would break these limitations down as follows:
1. External viewing/non-penetration - available to any certified scuba diver.
2. Penetration within 40m linear from surface, natural light, no restrictions, no risk of zero viz silt-out, no risk of entanglement, 2 divers able to exit side-by-side sharing air - available to any wreck speciality trained scuba diver.
3. Penetration in excess of 40m linear from surface, no deco, no natural light, some minor risk of silt-out or entanglement, inability to exit side-by-side sharing air - available to wreck divers with advanced training, high experience, team skills and specialist equipment configurations (long hose and redundant air).
4. Penetration in excess of 40m linear from surface, moderate decompression, no natural light, moderate risk of silt-out or entanglement, inability to exit side-by-side sharing air, moderate restrictions - available to technical (decompression) divers, with appropriate equipment (isolated manifold doubles and long hose) and basic level technical penetration skills.
5. Penetration in excess of 40m linear from surface, substantial decompression, no natural light, major risk of silt-out or entanglement, inability to exit side-by-side sharing air, major restrictions, highly complex penetration routes, water movement/flow in penetration - - available to technical (decompression) divers, with appropriate equipment (isolated manifold doubles and long hose) and advanced level technical penetration skills - i.e. full cave or technical wreck training.
There is nothing 'wrong' with any of these levels, or activities, or with divers who wish to dive at them. Everyone has different interests and personal 'comfort zones'. No level is more or less 'valid' or 'worthy' than another.
The only thing that matters, is that divers recieve the appropriate education, training and experience to conduct the type of diving that they wish to persue. That they apply common sense, honest self-appraisal and moral courage to their decision making..and do not allow themselves to be drawn outside of the level of diving with which the are comfortable and capable of doing...whether that is from external peer pressure or internal ego.
