There are always some small community left for each game, but there won't be a lot of servers to choose from.
Depends on where you live.
If you live in Western Europe you're golden: plenty of countries with dense population close to each other. Ping is never an issue.
Older Battlefield titles are still very much alive here.
Even Forgotten Hope 2 (= a WW2 full conversion mod for BF2) is still alive (and the dev team is still bringing updates). You can always find exactly 1 100-players server that's decently packed (60+ players) when the evening falls in Europe.
On the other hand, this is not the case in America, looking at comments on the mod's forum and the server browser.
I recently got into BF Bad Company 2 again.
Here an overview of the server browser with my preferred filter settings, filtered to Europe.
Captured on Sunday October 5th 2014 at 19:39 (7:39 pm) Central Europe Time.
Now the same settings but for North America.
There's a 7 hour difference between Central US Time and Central Europe Time (CET) so that would be around mid day.
Definitely not the best moment to capture the popularity, but it seems playing in North America at the evening also won't be an issue.
So Bad Company 2 is still very much alive.
Remember you only need 1 or 2 servers that are packed and have decent ping, to have a good time with any older game.
Regarding BF4's DLC:
I don't have BF4 but I assume you will be safe for the next 12 months.
Here are BF3's DLC servers that are packed in Western Europe, captured moments ago.
I assume BF4's DLC servers are at least double in amount. And I assume there will be less people switching from BF4 -> Hardline then there were BF3 -> BF4 switchers.
