If only one Bitcoin miner gets the block reward, is it worth mining only for the transaction fees as a (not-so-competitive) solo miner? I have heard (but not that knowledgeable) about mining-pools. If a weak system joins the pool, it has a chance to get a portion of the block reward, but I assume that not all of the miners are joined to a mining pool.
If I use my average PC for Bitcoin mining, it is a very low probability that I will be the first to solve the hash problem, thus, I won't earn the block reward most probably. I will only be rewarded with the fees with respect to the calculations I have made. So my question has two parts;

I assume that the numbers showing the average revenue generated from mining includes both the transaction fees and block rewards, is that correct?
If that is the case, then shouldn't the weak miners consider a lower average revenue, until they earn the block reward with such low probabilities?

Please correct any points that I might have been wrong about. Thanks in advance.
I will only be rewarded with the fees with respect to the calculations I have made.

That's not correct. If you don't mine a complete block, you get nothing at all.
Joining a pool will let you get a proportional share of the income (from both the block subsidy and the fees), changing the game from a lottery to a more predictable payout based on your contributed hashing power.