That trailer and the announcement was posted on the 31st of March, but they put April 1st as the date on the announcement.
I am fairly certain that this was done intentionally and that the trailer was a prank. However the trailer could also be for a second April Fools event component, as the WWI event is significantly shorter than the previous years’ April Fools events.
My thoughts (warning: many words)
That secondary event won’t be a test for player controlled infantry either, just for more AI infantry. Likely the infantry in that event will actually be a threat to tanks instead of just being decorations at best.
The infantry in the WWI event probably is a test to see if the servers and the player clients could handle large amounts of AI running around and doing stuff at all and since that seems to work just fine after some initial hiccups, a second event could happen as a proper stress test with more involved AI infantry.
If Gaijin really had plans of introducing player controlled infantry, the Mobile Infantry event likely was a test for that already. As you might guess, if sci-fi soldiers can’t really deal with tanks, regular soldiers wouldn’t even be remotely viable for the game.
You’ve got to keep in mind that Enlisted, Battlefield, Battlebit and whatever other Battlefield-like shooters are out there are made specifically with infantry in mind and the vehicles get a secondary role. War Thunder is the opposite of that, the gameplay is entirely focussed on vehicular combat, every aspect of WT revolves around vehicles and their interactions with other vehicles. Physics, game mechanics, game modes, maps, damage models, etc all are designed around vehicles.
Adding vehicles to a previously infantry-only game is, relatively speaking, a rather simple task, but shoe-horning FPS/TPS infantry into a game that was designed as a vehicular combat “simulator” would be a herculean task.
You need an isolated game mode that severely limits the main component of the game (the vehicles), you need special, entirely new maps with vastly more complex geometry (buildings with actual interiors), new control schemes, physics and game mechanics, a different matchmaker, hundreds, maybe thousands, of new models (soldiers, weapons, doodads, etc, even if everything from Enlisted was ported over), new sounds and hearing mechanics, new customization methods, etc, etc… All of that to create a wannabe Battlefield game within a game that people play for the vehicles.
I couldn’t wager how many paying players they would need to get into WT to break even with the investment, but I don’t think the entire Enlisted community and then some would be enough… And as I’ve said before, if other factors don’t bump WTs age rating up, playable infantry definitely will, meaning Gaijin would be have a much harder time marketing the game and (if that isn’t already the case) content creators would get hit with age restrictions as well.
Enlisted isn’t at risk of getting shut down at all, Gaijin keeps way less profitable games running and other game companies do the same. So far the only free to play games I’ve seen being shut down were either EA (greed incarnate) games (Battlefield Heroes, Battlefield Play4Free, Need for Speed World, Warhammer Age of Reckoning), games that were run/developed by companies that went bankrupt, defunct or dissolved or browser based games.
Most companies (especially ones that focus on the games-as-a-service principle) are willing to keep their less popular games running for a very long time. They just slow down active development up to the point where they only do the bare minimum to keep the game playable at minimum cost.