Your answer doesn't address the question however. Right now, under existing rules, your animal companion fights for you, and may die. Indeed, that's the major complaint, they die too easily. So discard any change in tone or alignment, discard my argument about using animal companions up more often, just as they are played right now under your argument it would be unethical to play a beastmaster Ranger. Just full stop, taking an "innocent" panther into the depths of a dungeon at certain risk of harmful and painful fighting and possible death, that is currently unethical and a good aligned character simply should never play a beastmaster Ranger.
You can see the problem with this perspective now, right?
I'm a vegetarian living in urban Los Angeles in the 21st century, and *I* am the one advocating animal companions were likely intended as more of an exploitable resource which frequently was replaced over a long rest.
I too think that frequently replacing your beast is one of the ways that the Beastmaster was meant to be played, and works well for neutral character alignments. Ultimately, if you choose to use your companion as a weapon, it means they will die fairly regularly, and the class is designed to not give you an easy out for that (though the UA ranger does). The same would be true to an even greater extent if you train your own companion. The fact that a trained animal acts on it's own would reduce it's survivability compared to the Beastmaster Ranger companion, and would be nearly useless past Tier 1.
But the other way to play the beast master is as it's true protector and friend. In this case, you should infrequently use the beast in combat, getting primary use out of it the way an Army Ranger might get use out of a drone, as a way to scout ahead and provide targets and the opportunity to ambush your foes. In this play style, the bonus's that the companion gets in comparison to what a PC who merely trains a pet are substantial, giving your companion much higher levels of survivability as it scouts ahead for you or for the occasional combat that it comes in to help out. At higher levels, it can help out in more combats per day, but still isn't supposed to be involved in every round (hence why at 7th level you get the ability to disengage, dodge and help as a bonus).
While not appealing to everyone, this second style of play, where you are the beast's protector (not wielder) for some is a huge feature of the class, not a bug. Having an animal to protect and worry about in combat is what makes it fun. This, of course would be the more good aligned way to play a beast master.
I do feel that some DM assistance is helpful for this style, such as allowing the beast to start all combats taking dodge and auto taking dodge if not give another action, or to start combat "off the board" such as the way Vex's bear in Critical Role did, so that a AOE doesn't take out the beast before it can act. But even with a tough DM, a player thinking protection for her companion first should be able to find ways to keep the worst from happening by being careful about where the companion is in the marching order and when and how it is used.
It takes a different way of thinking about how to play the class compared to previous iterations of it, and I'm wondering if as D&D has picked up more and more new players to the game over the last few years without the baggage of previous versions, that the WotC team has seen satisfaction level with the class and subclass rise.