Sentinels and pets seem to exist primarily as another grind. Their utility is simple, obvious and easily replaced or negated. There's almost no game play to them beyond things like "Bring Carrier if your weapon blitzes through ammo".
In principal it should clearly be worthwhile having companions that we invest in. Mods are well established and work quite well and giving companions our primary utility features (while terribly shallow) is technically compelling.
Decision makers need actual data on how different kinds of players use and gain benefits from their companions. As with everything else in Warframe, the further along a player is, the less they need worry about vulnerabilities (either of their frames or companions). Less proficient and less invested players inevitably suffer most and this only adds to the cycle of things that drive people away.
No matter what level you're at though, you have to admit that for most people seeing their companion (particularly a 'living' pet) die on missions, let-alone over and over again is disheartening and clearly shows the companion really isn't anything more than a vacuum or portable buffer.
There are much simpler ways to provide those kinds of features than creating a pet system that then goes on to constantly reinforce in people that (true in fact or not) the devs see your pet as not only expendable but worthy of being murdered over and over again for no reason, often to the point that they'll die and we'll only notice long after or we'll tire of resurrecting them and simply set them up every so often while on our orbiter and not give them another thought until we need to switch them over for some other minor utility.
As they do provide utility, it's reasonable that there should be some risk on using them on missions. As already mentioned though, that risk is extremely disproportionate. The companions of long-time players are effectively immortal in many cases, again, it's generally only the less invested and less proficient players who are penalized.
In the real world many people who have pets care about them. There's an opportunity in having companions of various kinds in a game to do something with that. I suspect the original intent of companions was aware of this.
In a statistical analysis (ie somebody at DE populating a spreadsheet with data gathered from how players play), it should be reasonable to balance companions so that most kinds of players have the same experience.
If companions are intended to be nothing more than stat sticks/vacuums/ammo bags then there's really no point in not making them invulnerable, or simply baking that feature into the game as standard. Giving people a useful utility feature that is taken away from them over and over again on every mission is really just a recipe to annoy them and definitively get people to treat the game with similar disrespect.