Not changing things on the fly seems good to me. If it's something that's designed to be variable from the start I guess it would still work pretty well (e.g. civics in Civ IV) but for the most part it takes away consequences. All strategy game decisions shown have consequences, and being able to change at any time means that you can avoid the long-term consequences. Also takes away the ability to do short term vs long term decision - would you rather one free tech or +5% to science? Would you rather +2 food per turn now, or 10% net food? Everyone would just take +2 till it's worse than 10%, then change. Takes away some decisions.
Optimal paths shouldn't exist in a properly designed linear path, as it depends on the initial circumstances. Sure, it may be best to go for tradition as your first policy in Civ 5 for the most part (though this might change with that latest patch), but if you're in an area with a massive amount of land and no AI, it makes sense to go for a liberty start (if properly balanced

). There's no optimum path because each game has a different start. If you had the same start, sure you could work out an optimum strategy and go for it, but the varying initial conditions means there is never one optimum path you can pick before the game starts.
(Sorry about Civ 5 examples - only finished one game of CIV:BE, don't want to use examples I'm not very familiar with)