1. The game can be played by 1-4 players. There is a little scaling with slightly fewer enemy patrols with less than 4 players. If anything, clearing objectives will just be slower with fewer players, but nothing is walled off, apart from an occasional resource bunker that usually needs two players to unlock, but you can get those resources on other places.
2. The devs try hard to give every weapon and support it's niche. The early weapons are great for low-armor enemies which appear at all difficulty levels, but they unlock anti-tank weapons early on. The later unlocks are just add variety to your play styles, and the premium packs, again, just add variety.
3. There's a lot of variety in mission types. The devs typically fix balancing issues based off community feedback.
4. The maps and missions are procedurally generated, but done very well. Apart from the mission areas, the maps never feel the same. Missions can range from running around a large map to get the nuke codes and launch a nuke, to defeat enemies in a small area. Most missions have optional objectives and enemy bases that will give more experience and resources if you clear them. Clearing a map feels great.
5. For high difficulty missions, you need some variety in light crowd-clearing weaponry and low-medium density anti-tank weaponry, and some in-between. A single skilled person can solo it, with sneaking and a lot of time. Opt for lower difficulty missions (5-6) if you just want to kill things without having to worry about packing for heavy armor enemies every few minutes.

Progression is all adding variety to your play style. Weapons don't often become obsolete, although you may favor play styles that favor certain weapons.

This is a game that'll be great fun and a unique experience for a few weeks. Then you'll be satisfied and done playing. The devs will be releasing updates while you play other games and when you come back, there'll be more content to explore. It is not something you will be grinding for 6+ months, but it offers a unique co-op experience that is refreshingly different from other shooters.