Richardo,
I don't agree with a lot of what you are saying, there are a lot of deficiencies in your workout. Okay I'll start from the top...
Bulking and/or gaining mass are not as easy as it sounds and usually the gains in strength come long before any mass is increased. There is absolutely no disadvantage to getting stronger. What this type of workout does however is usually every muscle or muscle group is worked on, decreasing the likelihood of mismatched muscle groups - decreasing the potential for injury.
The main reason why it is hard to maintain bulk after your 20's is because people don't often have the time anymore :work, wives, kids, family commitments, etc. stop most people from working out 6 times a week like we all did back in our late teens/early 20's. It also takes longer to recuperate from injuries, which is what the growth of muscle tissue relies on (at a microfibre level). Muscle doesn't turn to fat, the proportions in your body just change especially when you stop training and don't adjust your diet accordingly.
A training regimen consisting of pushups, pullups, running and ab crunches? Lets see, you are neglecting: Most of your upper back, 2/3rds of your shoulders, lower back, most of your abdominal muscles, hamstrings, forearms and I guarantee that your pecs will lag if you rely on pushups, for many people, these primarily work the front delts. You are ignoring whole muscle groups and THIS is what will cause you injuries.
So far the only part I agree with is that running should be a part of everyone's training in some way. But this will not get you looking good or build real muscle in the legs, with the possible exception of your calves (ever watch the Olympic marathon runners, they are probably all 55kg max!), it will however increase your overall fitness which is good.
If you changed the 100 crunches to : 25 straight crunches, 25 crunches each side and 25 leg raises, that would be a more complete ab workout. Just 100 crunches would help you get the 6-pack but would leave you with poor muscle tone everywhere else around the middle.
Basically all your workout is going to do is make you good at pushups, pullups, crunches and running. This will not make you look like a model. In fact I think that if you were to go through with this routine, you will actually end up looking quite strange.
Go back to the drawing board on this one, this workout is not well thought out.