"c'est évident: j'ai toujours fait comme ça" Bastien always do so and still doing so at the moment he speaks,

It appears your question was never answered, so here goes. It appears you are talking about when an action "has been" going on "since" a particular time and continues into the present. [9A1 in Promenades.] Two points: 1) Bastien does not say since when he has been doing that. In fact, 2) he says "always" which doesn't mean any specific start time. So, had he said, "I have been doing that since last year", then you would use present tense and "depuis".