I believe that the best time to start buying ASIC miners is when the next Bitcoin halving is not very far away.
I said this because I've seen many people did this with both Asics and GPU miners and they later sell their equipment when the next bear market is almost here because equipment starts losing value again, it's like a never ending circle.
I'm not really sure why you said buying when the next halving is not far away is the best time. Are you assuming the profits for the next few months would be massive and allows them to get back their investment so quickly?
As far as I know, there are many factors that can influence your buying decision, including whether you have access to cheap electricity or not. If you do, anytime can be a good time to buy mining rigs. CMIIW.
My actual question is, what will happen with Bitcoin mining in future? Will this ever head one day? because blocks reward getting shrink every circle must end on day, or am I not getting a part about Bitcoin mining?
I suggest you search for discussion about block rewards or mining rewards in this sub-board, or Bitcoin discussion. Your worries have been discussed many times before, you might be able to learn something new from those threads. In short, the market will always look for equilibrium. Miners will come and go, difficulty will always change to balance that out.