It will degrade over time for sure, but the recent series of downspikes is from a multitude of things.
People were getting weary of cex.io/ghash.io having the market share on mining and harnessing 51% of that power can be a bad thing as they could potentially control the blockchain for a period of time. This is from the wiki:
An attacker that controls more than 50% of the network's computing power can, for the time that he is in control, exclude and modify the ordering of transactions. This allows him to:
Reverse transactions that he sends while he's in control
Prevent some or all transactions from gaining any confirmations
Prevent some or all other generators from getting any generations
The attacker can't:
Reverse other people's transactions
Prevent transactions from being sent at all (they'll show as 0/unconfirmed)
Change the number of coins generated per block
Create coins out of thin air
Send coins that never belonged to him
It's much more difficult to change historical blocks, and it becomes exponentially more difficult the further back you go. As above, changing historical blocks only allows you to exclude and change the ordering of transactions. It's impossible to change blocks created before the last checkpoint.
Since this attack doesn't permit all that much power over the network, it is expected that no one will attempt it. A profit-seeking person will always gain more by just following the rules, and even someone trying to destroy the system will probably find other attacks more attractive. However, if this attack is successfully executed, it will be difficult or impossible to "untangle" the mess created any changes the attacker makes might become permanent.
https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_powerCheck out
https://blockchain.info/pools for a pie chart. Also:
https://en.bitcoin.it/wiki/Comparison_of_mining_pools for more info.
People moving away pushed the value/price down and that drove more people away, like a downward spiral. Still in the middle of that now.
In the end it is bringing the price down for hashing power, which is good for you.
Check the price against some other options:
https://cloudhashing.com/contracts$1000 for a year at 20 GH/s
https://products.butterflylabs.com/homepage-new-products/1-gh-cloud-hosted-bitcoin-hashing-power.html$10.83 per 1GH/s for a year