True, many people start or end their Lightning journey by using an exchange. 

That makes me wonder whether or not apps will take over as the largest nodes on the network once a circular economy of earning and spending really takes hold. 

I guess exchanges also offer node operators the ability to re-balance channels without incurring Bitcoin network fees, but maybe apps can do that too.

I know of one well-capitalized individual earning $3-5k per month in routing revenue on Lightning, but I don't think there are many examples of other individuals replicating that level of success yet.

If individual node operators don't end up earning much on the Lightning Network, what role will they play? How important is it to have a flourishing network of individuals operating nodes?

bitcoin

Exchanges on the Lightning Network

I don't see any risks, if anything it makes sense that exchanges manage most of the liquidity because they're probably either source/destination of large txns.
I've never understood how/why hobby node operators would earn fees routing payments. At least not yet anyway.