First, hello and welcome to the forum.
There really haven't been any major scientific breakthroughs that have changed the hobby but there have been changes. Before the mid 90s, few people, if any, had ever heard of fishless cycling. As mentioned, people set up tank and introduced fish slowly until the tank was full. But the biggest difference between then and now (at least in my opinion) is that not a lot of people were in the hobby. We didn't have the major chains like Petsmart, Walmart, Petco, etc. selling fish so that they were so readily available to anyone and everyone. So the people that had tanks just cloned one to start another and thus, no cycling needed. In thinking back (I'm 55), I really can't remember ever seeing aquariums very often when I was a kid and almost no place sold fish (trying to remember what store it was that I loved to go in because they had fish for sale). Before say 1980 or so, I wonder how many people tried keeping fish but gave up because they bought too many to start with, didn't know what to do when problems arose and just got frustrated and got out of it as fast as they got in. They certainly didn't have the internet to turn to for answers.
You did mention 2 things though that make a huge difference in not having problems though: plants and helping the bacteria from an existing tank. I already mentioned cloning a tank and that basically eliminates any cycling. If you take half the filter media from a 50 gallon tank and start another 50 gallon tank, you could fully stock it immediately too. The bacteria in each tank would double in a day or so and both would be fully cycled. And plants can use the ammonia in its original form so you could probably fully stock a heavily planted tank and not really have much, if any, problem with ammonia at all as the plants will take care of it.
There are a several reason fishless cycling has become popular. First, it prevents subjecting fish to toxins that, even if they don't kill them, can still cause damage that shortens their lives. As a society, we are less tolerant of animal cruelty (and some people would argue that "it's just a fish") so most of us in the hobby want to be a humane as possible. Second, it prevents you from having to buy hardy fish like danios or goldfish, fish that you really aren't interested in keeping, just to cycle. You are left having to "dispose" of those fish in the end. Third, it allows you to stock your tank fully as soon as it's cycled. No more adding 2 fish, waiting a week or 2 and adding 2 more, etc.
Fishkeeping is more popular today that ever before. We can thank the chain stores for that. While we curse them for having terrible conditions in their tanks and giving bad advice, they have exposed more people to fishkeeping. On a regular basis, all of us are in stores that sell fish but are not "fish stores". Unfortunately, too many people buy them as an impulse buy and there is where the problems start. They buy a 10 gallon tank and the "fill in the blank" pet store employee sells them 3 goldfish, an angel fish, 6 neon tetras and a pleco to start with. And a week later their fish are covered in white spot and dying and they wonder what went wrong. So last but not least, the best reason, in my opinion, for a fishless cycle is to allow beginners to get into the hobby with the least amount of loss and work. It's a whole lot easier to test ammonia and nitrite for 3 or 4 weeks than to do water changes every day.
So don't some in here trying to tell us how to cycle a tank.
Only kidding.
Once again, welcome, and enjoy the forum.
