Why does everything have to be a renewable resource? Starcraft has crystal mines that run empty. I don't see why Marines even need to be replenished. You build a Cloudsave and you go forth and capture a 10k value Mothership from the enemy. I would say...great! Go you! A good investment and outcome for those marines. For under 1k for the transport and your 1-shot marine attack, you netted +10k value for you, and -10k value for them.
Why do you feel like you should be able to clone more marines and do it over and over again?
Actually you can... you will likely lose ships as you fight and each new one gets another marine-assault capability. You can even sell a ship and recreate it (though at heavy cost) if you were desperate to stop a big attacking ship.
Now, they made the MarineCloner. I don't really like it exactly, but they DID listen and implement something to address the issue. It is not a perfect system, but being able to trivially create more marines out of thin air would break the game balance.
