I think your concerns on these two issues are right on, meaning they correctly identify the challenges Steemit and Dtube face currently. But, it's also important to keep in mind that there are other projects being built on top of the Steem Platform to address exactly these very issues.

Regarding content Discovery.-
We currently have: dmania, steepshot, zappl as well as running ICO built on SMT's (smart media token). Being these, let's say beta initiatives a sign of things to come, it's obvious, or at least it is to me, that eventually this will lead to the definition of niches and communities.

If we add this this the fact that one of the upcoming forks has the intention of building communities within Steemit, I don't see this being much a problem for too much longer.

Regarding the Continuation of Rewards.- I guess this makes sense if we think of the "money" we make on Steemit, with the same mental framework with use when we are talking about Fiat Money.

I'll be specific... If I make an amazing video on Dtube, that gets I don't know a $1000 payout. Let's say for the sake of making an argument that at the time I made the post I collected from it 400 Steem tokens, and each token was worth $1 USD.

If, I simply keep the Stake on my wallet, meaning I only cashed out the SBDs from the post and keep the STEEM, a year later my 400 Steem tokens could be worth 20, 30 USD. So, my video has made me 8000 USD in a year, or more. On top of that it also gave me influence in the platform allowing me to interact with other users and earn more rewards in different ways, so the number that that 400 Steem Investment has paid me out is very much underestimated by my calculations.

One the flip side of things, I believe that it can also mean that users stay engaged in the community and don't simply post one homerun and run. Granted it seems like more work, but I thing it's a net positive. A content creator is highly incentivized to never stop putting out content, thus also adding value to the blockchain at all times.

This is all to say that depending on the type of content it may or may not have an expiration date, a video about how a whale gives birth to its young, does not necessarily expire, but something political might.

I'm not blind to the notion that these are not "perfect" solutions, but I will be inclined to say that they are currently (note the word currently) better than youtube, and more importantly level the playing field a little more for smaller accounts.

It's close to impossible for a brand new youtuber to succeed on youtube in a short period of time, it's not entirely impossible for a new content creator to make enough to pay some bill on steem within the month.