One issue that affects the longevity of Career Mode is that players aren't replenished adequately. I'm in my 10th season of career mode, and I took over the German national squad in my 8th season. Several of the old guys are still around, like Muller, Goetze, Draxler and a few others, but there aren't very many high quality youngsters coming through to replace them. There are 2 or 3 regents that are quite good, but after that I have to fill out the squad with players rated in the 70s. The German national squad should not be relying on players in the 70s.
I think the problem lies in the regent system, and the random players who do not grow. When 3 of the best Germans retire, 3 regents are created, but only 2 of them become decent players. In the long run this makes for a lot weaker talent pool then there currently is. In my career, the Lahm regen was a dud and now Germany have no fullbacks rated higher then 75.
The only saving grace to this situation is that I started youth scouting in Germany right from the beginning of my career because I knew I eventually wanted to take over their national team with Fifa 16 coins. Because of that, I have enough quality to choose from. That isn't a very good fix to the problem though, because every other national and club team who I'm not running is suffering from lower talent.
What needs to be added in is a youth scouting system to every club team. If clubs all started off career mode with their own ability to scout local talent, and one or two youth players got promoted to the professional level each season, the talent level would be much better. This would be way more realistic, and would counteract the issue of players that randomly do not grow.