As pol-bear states, for nearly all longer distance trains now the 10 (or sometimes even 20 eur) supplmt that includes the obligatory REServation for TGV and most IC-trains has to be paid for too. Depending on your travel plans indeed fixed advance fares/bookings may turn out wiser. In the Paris area, though you are entitled to use all SNCF (but ot RATP) trains, the barriers and need for tickets to franchise these can also be offputting.
Strikes in FR usually do not hit the whole of the network, nor all trains, it goes by union, of which there are several (though none is said to have more as 10% of staff), in most cases you can use other trains, but there is never no garantee. Unions compete in how eager they are to call strikes-but hardly ever all at the same time. Also even though these nasty strikes (often reminding the state of the old UK before a mss. Thatcher came in) often start much earlier and last much longer as ''advertised'' due to the idiot complicated rosters of staff. On local lines SNCF often lays on replaceing buses instead.