I would like to disagree. Since the release of duels on 1.8 there are 50+ players on at a time, usually there are at least 60 players playing ffa and I think the majority of Skywars players play on 1.8 (or at least a few hundred), and there are lots of eggwars players as well. You mention that the 'already small playerbase would spread out even more and make games fill slower and feel emptier.', however I feel like just less than half the players play on 1.8, and there are only half the games on 1.8, so I think it would do just fine if it were added.
Looking on the server myself at peak times, FFA had around 80-100 people on which is slightly higher than your estimate however this is still only just enough to fill 2 FFA servers and a bit, which is very few considering how a lot of these people won't move over to assassinations as the majority will remain in FFA.
I also don't know where you pulled the 50+ people on duels figure from as the number counts on the NPCs are 1.8 and 1.9 combined, but considering how the ratio of 1.8 to 1.9 players is around 1 to 6 (for every 1 1.8 player there is approximately 6 1.9 players) then we can estimate that there should be around 14 people from 1.8 on duels (based on the around 90 peak players I saw duels have), although this number is likely very slightly higher due to the recent release date of 1.8 duels, however this is still extremely insignificant.
Your statistic about there being "at least a few hundred people" and "the majority of skywars players play on 1.8" are also extremely false. The numbers above NPCs heads are for 1.8 and 1.9 Java combined - not 1.8 alone. There's also no way that it could possibly add up anyway, as the server peaks at 600 people. If there was at least 60 FFA players, and 50 duels players then that's 110 people already. A few hundred people on skywars would be at least 300, and considering how eggwars is more popular than skywars, that would be at least 350 on eggwars, and how lucky islands is neck and neck with skywars, another 300 players. This totals over 1000 players - which is nearly double the actual player count that 1.8 actually gets at peak.
There quite definitely is not the playerbase to support assassinations. There might be enough at first to fill up a server at peak times, but during lower times and not long after release it definitely will not have the playerbase to support it.
This is also leaving out the fact that developer time would have to be spent doing this, when in reality you really need to be focusing developer workload on the other parts of the network which are far larger and have far more potential.