I feel is that they should keep the cost of making a Titan relative to their experience when being made. Not only would it justify the retention of the experience by Titans, but it would also make you protective of your high level titan. Not too much cost variance, but still should be there. Like, in one game I was playing, I was on the verge of destroying an Advent rebel empire. They only had like 3 planets/asteroids left. I destroyed their level 6 Eradica (it took down a lot of my ships before it died, damn invulnerable power ). But then their allies attacked me with a huge fleet, so I had to retreat. I built up my fleet again, BUT by then they had built another level 6 Eradica. With their 3 colonies, their economy should have died. But here I saw another poewrful Eradica, who again took down ships (less this time, had made more carriers and bombers). Their allies weren't helping my cause either.

I'm not saying cost scaling is absolutely necessary, but just an idea. It justifies retention of level and XP.