On one hand I support DropBox, because they care about my needs - I'm (also) a Ubuntu Linux user and Google apparently doesn't care about me enough to provide me with a client. Apparently I'm so unimportant to them that they can't justify the cost of a couple of man-months.
On the other hand your worry, while justified, does not apply to Google Drive. Google Drive is now an integral part of:
- GMail
- Google Apps
- Android
- Chromebook
- Google+
Yes, they probably don't make much money directly from neither of them, but these represent Google's
platform, or lets say Google's own Windows if you will. It's their way of achieving lock-in and protecting against competition that may harm them, as otherwise Google's Search engine which drives the real money from advertising - cannot achieve lock-in on its own. And each of those products need cloud storage and cloud storage is becoming the norm. Microsoft has SkyDrive, integrated with Windows and WinPhone. Apple has iCloud, integrated with OS X and with iOS. Clearly you can see a pattern.
With Google Cloud the only thing I don't trust long term is their pricing - they have a history of releasing cheap or free stuff, earn popularity, earn users, after which they realize that their costs are greater than their earnings and raise those prices. Examples: Google App Engine and Google Apps.
In regards to DropBox, I support them but they really do need to reduce the pricing for those paid accounts. Pretty soon I'll need more than 100 GB and paying $19.99 per month for 200 GB is a little too expensive already, given that after a while I'll need more than 200 GB and then for 500 GB I'll need to pay $50 / month and that's way too much, when Google's 1 TB offer is for $10 / month and would last me for 2-3 years at least.