I'd say more players have been lost due to lag and disparity between devices than to any other factor. To me, if there was ever a "golden age" of DF it was when the game was only available to mobile devices. Most players were on an even footing, no one seemed to have much of an advantage over anyone else. When PCs joined in there were huge discrepancies, they have been adjusted here and there along the way but that was the first time there was something that trumped skill in game. Still at the time the game remained busy and popular because Zup was heavily promoting the game. Remember the offer of a free D8 for anyone who could sign in enough friends? This was the time of the fastest growth for the game, when GI, Wolf Pack, Lafayette, TFL, and many smaller squads sprung up. Lots of them (not all) were PC players. Do you really think promotions don't play into the game popularity?
Lots of people loved the D8 but pehraps just as many hated it. It gave a huge advantage to the German side and it got hard to find games that had a fair number of players willing to fly Allied. For Kindle Fire it was worse because the updates were something like 6-8 weeks behind. Others were able to play with the (small hit zone) D8s but you couldn't get it for Amazon devices. Even without the device disparity the D8 had such an advantage that Zup ended up pulling it.
In Elite, Zup had to keep adjusting the control sensitivities. Sometimes the PC was untouchable, sometimes the IPad was the top dog... When the PCs got nerfed and were easy prey to tilt devices that's when a lot of DF1 PC players didn't make the switch to Elite.
The bullet drop was not the only factor that made shooting more difficult, at the same time Zup also made it so planes were not just a solid block of hit zone. Now you actually have to hit the plane not just the general area, bullets can go between wings etc. It gives head on shooters a much bigger challenge that players find very frustrating. I believe the problem with aiming is mostly due to lag. With lazar shooting there is less info for the servers to have to process and anticipate before the signals are far transmitted all around the world between different devices. Even now there are some games with very little lag where the shooting is much more predictable, then a new player can enter and everything goes to crap.
If Zup put out a new add campaign there would be a big influx of new players because advertising works, whether or not they stay is a whole different matter. I have not seen a single post on this thread (or any other) where anyone considers what the economic and management impacts are of having a whole bunch of old-school high-ranking players vs. having a steady stream of noobs... what is more time/cost-effective: having lots of players and paying for lots of premium servers, or scaling down and having less game income but lower overhead expenses? Don't worry, you don't need to think about it, but I can guarantee that Zuperman weighs every decision with the bottom line $$$ and with consideration of how much time and energy he has to put into managing the game, including upgrading his hobby to get closer to his vision of what he thinks DFE should eventually become.
If you want to wax poetic about the good-old days, pick a date, pick an update when you think everything was perfect. Lol. There has always been someone complaining about the changes.