calculations require the knowing of the load. If i have a mechanical relay with
250V/16A i can switch everything and I'm flexible - no matter if its inductive load or not,
Not true, you need to know the current an if it is AC or DC in order for you to know if your relay contacts are rated correctly. Same goes for inductive loads, you need to know what spark quenching components to use.
If you choose to ignore this for relay contacts you can have the same success as with a triac.
You mentioned a triac, those are AC only devices, that is if you want to be able to easly control them to turn off.