There seems to be a common thought that seed catalog companies are growing their own seed. It also looks to me that the catalog companies, themselves, encourage this notion. I suspect that the
reality is that they grow very, very little generally.
First of all, they are in the retail sales business. That, in itself, must take a lot of corporate resources and focus.
Next, imagine that a particular hybrid is a popular favorite. We know that a hybrid is the product of 2 (or more) different parent lines. Who owns these parents and who is doing the hybridizing? It isn't likely to be the 10 or 20 seed companies that are selling the hybrid seed. It is one seed company that is paying folks to walk around with paintbrushes and bags, spreading pollen and isolating flowers. The product of their efforts is being distributed thru catalog sales.
If you have a favorite variety, hybrid or otherwise, and Monsanto buys the seed company that produces it - what are you going to do? You can take a principled stand and refuse to ever grow that variety again. Maybe I haven't thought this issue thru but I've continued to buy some seed even after ownership has been snatched up by Monsanto. And, consolidation in the seed "industry" has been the name of the game recently, just like in so many other industries.
If you go entirely with heirloom seed, you gotta admit that there were reasons why some varieties fell into disfavor. Maybe it was the processing, storage, and transportation methods of the food industry. But, some of the reasons for obsolescence apply to you, the gardener, rather than the food industry.
Often, new varieties have been better adapted to the growing conditions of certain areas of the country. Heirlooms can be very restricted in their performance. It isn't always the case. I mean, if the Brandywine tomato could only grow well in the Brandywine valley of Pennsylvania and northern Delaware, gardeners elsewhere wouldn't have much interest in this variety.
Adaptability is important to my gardening success. I grow some new varieties, some hybrids, and some heirlooms. In fact, for the last 15 years, I've grown a family heirloom tomato my grandmother grew during the Depression. It is one of 6 or 8 tomato varieties I grow. Fortunately Grandmother's tomato is happy here, nearly 2,000 miles from where she grew it and hundreds of miles from where my uncle kept it going for several decades.
I ate one of the first 2 to ripen on the plants just this morning! My wife had the other one. It is her favorite tomato (she likes them on the mild side) but her family wasn't even in the United States when Grandmother was growing these in her garden.
There's some "diversity" in my gardening, I'd like to think

.
Steve