That's why I said we need to improve abilities rather than raw stats. Being able to learn new abilities would be more realistic (known as cross training) so there is still soldier growth. For example, Stealth movement will reduce the chance and distance of being spotted, heavy weapon training will reduce the TU needed to operate hvy weapons, scout will get a spotting bonus, electronic skill can offer bonus to remotely operated vehicles (say more TU for a robotic unit). Instead of raw stat improvement, you can use proficiency improvement so experienced soldier get a TU bonus since they can do tasks faster (which is also more realistic). All that combined will make your soldier unique and valuable to keep alive.
More shots to kill is not really a problem if suppression is taken into effect. The amount of fire being taken (number of rounds instead of actual damage received) will affect the chance of a unit to stop their intended action and try to seek cover nearby (maybe with an exception to robotic units). they should also incur an accuracy penalty as it's going to be hard to line up a shot while being shot at.