I just don't see the benefit from a cost/simplicity standpoint. My calf crop isn't inconsistent. I have a type and end weight of cow I like to buy - but I'm pretty free to change that given that I'm not reversing years of breeding if I do. I'm using the same bulls on everything (except heifers) I can also buy up a year if I buy breds. There's lots of benefits.
You gave the example of keeping 30 heifers back out of 70. Assuming you had equal number bull calves you kept about 21% of your calves. You have to select the traits and breed of the bulls you're using based on that 21%. The more traits you're selecting for the harder it is to move the needle. You may be sacrificing on 79% of your calf crop. As you limit bloodlines, you limit heterosis too so you could be losing ground you gain.
I don't really see one way as right and the other as wrong, it's a preference. I probably have a higher cull rate(not a good thing). My calves will have more variability but likely gain better. Yours will be more consistent - you equate that with success so you're doing things the right way for you.
I agree there are benefits to a terminal crossbreeding program. For my operation, I personally believe that the benefits of a continuous crossbreeding program outweigh the benefits of a terminal program...but that's just me.
Some of what you said is spot on, some is a little off.
You are correct that you will have higher hybrid vigor in the CALVES in a terminal cross program than a continuous program will. Your retained heifers and your cow herd will not (since you are not retaining any), and there is a lot of benefits to having heterosis in your cows as well as your calf crop (it's about a lot more than simply lbs gained).
Your statement about limiting bloodlines and it directly limiting heterosis could be true...it also could be false. It depends on the management of the rotational crossbreeding. A person keeping heifers and swapping out sires from different bloodlines could end up having less heterosis than someone doing a true rotational breeding program of 3 breeds. Let me try to clarify:
Heterosis is the inverse of inbreeding depression and assuming a person has a high inbreeding coefficient in their rotational breeding program...then you would be correct...a high inbreeding coefficient would lead to a decrease of heterosis. However, in a true rotational breeding program, the inbreeding coefficient is low. Well below the 12.5% threshold established for performance. Essentially you use the same principals as line breeding a purebred herd, you just add the crossbreeding rotation. Wye angus has had a closed herd for 62 years. Their inbreeding coefficient is around 6%. After a while of running your own crossbreeding program you can breed your own replacements and still limit inbreeding and have good heterosis. It takes discipline and hard culling, but the results are worth it in my opinion.
Running multiple breeding groups is easy for me with multiple lease grounds that I run my herd on. The bulls/horses stay here at home, the cows go in their groups to each designated lease ground. Then I drop the right bulls at the right place for the breeding season. Doesn't really add much extra work for me. Even if I didn't have lease ground, my owned ground is divided into multiple pastures, which I think is a must have practice for grazing. Side note: Ranch size isn't really an excuse. A friend of mine runs cattle on 2 ranches which combined are about 89k acres. I want to say his 50k acre chunk is divided into around 212 pastures but I'd have to go back and look at his stuff. They are divided with single strand high tensile electric. He runs both cows and yearlings, and he has 2 "interns" that work for him...that's it. It's a pretty awesome thing to behold how he manages and rotates his cattle and how easy it is for a man in his mid 70s to do...anyway...back from my rabbit hole...
Most people probably don't have the desire to do a true rotational 3 breed crossbreeding program, and those that do half the time select the wrong bulls. If you want consistency and health you've got to avoid extremes in any direction. If you aren't breeding your own bulls you need to select bulls that fit your environment, fit your herd, fit your program, and fit your goals (that means actually writing down a vision and real attainable goals for your herd).
As far as buying replacements...that works for a lot of people...especially if you don't have the discipline to run a continuous crossbreeding program. I've bought replacements from friends and also from the sale barn, and I've been very selective about what I buy. Still, it's been my experience that no matter how selective I am with my purchased replacements, they cost me more in the long run than my retained heifers do. Short run sure, they are better. But if I have a retained cow that produces for 15 years with no calving issues vs a purchased heifer that lasts 5, the numbers pan out in the long run for retention. If you're management is right...it's far lower risk as well...but if your management is wrong then it can be higher.
The comment above about it depending on your herd is correct. I've got the traits I want in my herd and I want to enhance those traits. Bringing in replacements from outside takes me back in the other direction. It depends on you and your program.
Like most things in life, you get what you put into your cows. Some people put in a lot of time into their cows, their calving, fighting against nature...and they get cows that require a lot of time. I'd rather put in the genetics that save me that time so I can have a better herd and time with my family (well unless the wife is on a rampage in which case I'd rather spend more time with the cows).