Affinity, including shared affinity, is always converted to focus on lensed + maxed weapons, regardless of whether or not you have a Convergence active.
This is false. When a boost spawns, it appears uniquely and individually for each player that is capable of collecting focus with their current loadout (at least 1 lensed, maxed weapon). i.e. You cannot 'steal' or even see other players' boosts. If you get one, so does every other focus collecting player.
Perhaps a valid criticism, but it's not that simple. You could run away from your team, yes, but everyone on your team w/ focus is also seeing that booster. It would be at least as reasonable for you to work with your team to get to the booster, or to go to the booster immediately together as a team. In fact, there are more incentives for doing this which I will explain.
Now that the focus is shared among squad members via the shared affinity, and because the boost has a limited duration, you will, in the long run, gain more focus by having a squadmate with you than by doing it alone. You actually sabotage yourself by playing selfishly in the manner that you are suggesting is the efficient way to play. More tenno = more kills in less time.
The more players you have in your squad w/ a lensed + maxed wep, the more likely a boost will spawn. The more you stick together and play together to clear enemies etc. the more likely a boost will spawn.
Furthermore, the spawn position of the boost cannot be predicted. It is based on player positions, which simply means this: if you separate from each other and go off on your own, the boost will spawn in awkward positions for almost every player in the squad, thus making it a hassle. If you stick together, it will consistently spawn within a reasonable range of the entire squad, for everyone.
There is also yet another reason why going off on your own sabotages both yourself and your squad: so long as a boost is either on the ground, or currently active on someone, the likelihood of another boost spawn remains minimal. So if you went off on your own, and therefore caused the boost to spawn in an awkward position such that it takes far more time for other squadmates to get to it (or to even bother getting it at all), you would have literally added to the time it would take for another boost to spawn again.
I hope that this information helps clear things up for you, and I look forward to reading your constructive criticisms.