I've found PC flight simulators to be more of a hindrance than a help at the ab-initio stages of instruction, as student pilots tend to focus too much on the instruments than looking outside visually for the correct attitude. This might explain why it's taking longer for students to solo these days compared to the pre-computer age.
An interesting personal opinion. Far from finding PC synthetic trainers a hindrance, I believe that several hours practicing flying on a synthetic trainer allows the student to be more comfortable in his knowledge and potential skills before he does his ab-initio training in the real thing. Even basic R/T can be introduced while in the synthetic trainer or PC trainer since R/T is such an important integral part of early training. The PC trainer is ideal for training procedures the student will use in the air.
The "feel" of a synthetic trainer will be different of course to the real thing but that is easily overcome when the students starts flying. It is up to the instructor to then refine the procedures the student experienced in the PC trainer. While others may have differing viewpoints, having taught students in synthetic trainers before first flight I did not find they were heads down into instrument flying. On the contrary they were quick to rectify airspeed, compass and altitude deviations since they were used to scanning. In turn they devoted more time to the outside view and situational awareness since a quick glance at a performance instrument was a natural thing for them. More than anything, use of a synthetic trainer or PC trainer gave the student greater confidence which showed during ab-initio training in the real thing.
It is difficult to understand your point of longer time to first solo experienced by students who have practiced flying their PC trainers. I believe times to first solo started to increase many years back when all over grass airfields gave way to runway operations. In those days the aero club training area was typically only a few minutes away from the aerodrome which meant short transit times. My log book for example shows the majority of dual ab-initio instruction flights before first solo of 40 minutes and certainly less than one hour. Nowadays we see long delays before getting airborne at capital city secondary airports such as Essendon and Moorabbin and training areas sometimes more than 25 minutes transit time in each direction. The training environment has changed significantly. Of course frequency of flights, and student and instructor ability all come into consideration when looking at why dual hours before first solo have increased from 8-10 hours to typically 15 to 20 hours and even more. There is no shortage of anecdotal evidence that "hours building" by some flying instructors also means an inordinate amount of dual goes into the students log books in relation to solo time. In other words supervision of instructors by their CFI's is lax.
All of the above considerations must be taken into account when looking into the disparity in time to first solo in the old days when compared to present day flying training. I believe it has nothing to do with the fact that students today have access to PC flight trainers.
Re competency based training as another perceived reason for longer times to first solo. More dual hours does not necessarily produce a better student pilot. The skill of the instructor is an important factor. The mere fact the amount of paperwork an instructor is required to fill in after each flight in order to tick every box on the progress report, has increased four-fold from the old days, has nothing to do with time to first solo and certainly does not increase student handling skills.