Values : 1-Very Low, 2-Low, 3-Medium, 4-High, 5-Very high.
Probability X Impact =Result
51% Attack - P=3, I=5, R=15
Probability of a 51% is much lower than you think, it nearly is nonexistent. first of all because of the high difficulty and the huge hashrate that bitcoin has, you will need a very large hashrate to be able to attack bitcoin. it won't be 51%, you actually need a lot more. and that only works IF you can build a longer chain and get others to build on top of YOUR chain not the previous that you orphaned.
even if this costly operation is successful it can not damage bitcoin, you just switch to a bigger number of confirmation requirement and simply eliminate the risk of double spend attack. in other words it is impossible to double spend a transaction that is 3 blocks deep and in case of a 51% attack or in case you are accepting a large amount of bitcoin you simply switch to 6.
i would give this 1 * 1 = 1
Anti-Bitcoin Regulation - P=2, I=3, R=6
the result of this is not going to be big. because bitcoin is global and many big countries have already accepted bitcoin and regulated it to some extent (positively) and one country doing some crap won't affect bitcoin at all.
Protocol Bug - P=1, I=5, R=5
depends on the bug! bugs are fixable!
Domination by company - P=4, I=3, R=12
doesn't make any sense!
the probability of it is nearly zero. and if it happens bitcoin simply dies so the impact is the max because no one wants a centralized bitcoin and that is why the probability is zero.
losing private keys - P=1, I=5, R=5
that is the idiocy of individuals and the probability of it is high but the impact is zero because you are talking about some individual(s) losing their private keys and the effect of that on bitcoin in total!!!