@patch
Maybe highlighting the point about how many essentially momentumed creatures was bad. Here's another view, with your idea I can make a deck that uses quinted growth creatures(lava golem, fire spirit, etc) and for 3 give 2 of them momentum making for 3 a turn a strong 2 monsters and average 1 that would never die. The deck would most likely be made where such an upkeep is not a problem. My idea just gives you one 4 attack creature that will never die. My idea might not be great but seems much less OP to me.
First, if the opponent has thorn carapace, then fire spirits are likely to die because, if you have two quinted creatures and an immortal out, then each quinted creature has a 1/3 chance per turn to not be momentum'd.
Okay, so let's say 6 fire spirits, 4 immortals (these two would already take 20 pillars/pends to play the immortals and fire spirits as you draw them and to power the immortals' and the fire spirits' abilities once per turn), 6 quints, and 12 pillars/pends.
Not only is this type of deck extremely slow and has virtually no defense, but it is also pretty choked on quanta. In addition, as I stated above, the creatures are nowhere near invincible.
Your idea would make immortals much more OP than my idea simply because your idea is in no way balance-able, having one card that deals damage and cannot die no matter what is just like having an Aether card that costs 6
to inflict 4 poison, where the damage is hindered by shields. If you're going to increase the cost just to add this little flair, it'll only act as a nerf on Immortal, and you might as well make a new card with that ability that can be quinted.