1. Pretty much every single team out there will have money to throw around. If you want to try and land some of those star players you have to make this team look more attractive, else they should make the easy decision and join teams that are already set with talent/star players in their rosters and now somehow find themselves with money to spend because the salary cap increased substantially.
As wdleehi and Roy already pointed out, players of Middleton's calibre will not attract any star players. Our team will only attract star players if they feel we can offer them a better situation going forward than our competitors can (you acknowledged that).
2a. Players of comparable talent/potential should be more costly in the following years. Heck, even lesser players might be more expensive.
2b. As those players will be most costly, having a player on a better cost controlled contract (and young) will add another asset in potential trades to acquire a talented player if that's the road we're interested in going.
2c. Considering that the incentive to trade players will be by my estimation at an all-time low, having players under contract that you can actually trade, should be of benefit to us.
All of this assumes that 3rd year player Middleton will at least keep up his current level of production. That is not certain, and in fact, is the reason why I mentioned the cautionary tale of Ben Gordon (and I could've substituted the name Gordon with many other players). A player on a multi-year deal making 15% of the cap who doesn't live up to that sum is not as easily tradable as you make him out to be.
3. I mention that flexibility is not an issue because even if we make this decision we'll still have flexibility.
There's the salary floor to take into consideration as well. It'll jump to $81 million or so in 2016, next year should be about $60 million. Could be filled with 1 year contracts of course, but I rather take a chance on young potential than do nothing and miss out on just about every opportunity out there.
A player making 15% of the cap is still a player making 15% of the cap, no matter how high the absolute cap ultimately is.
All that said, in 2016 the only player I'm actually really interested in offering max dollars to, that could be attainable, is Kevin Durant and you're not attracting Kevin Durant by being a team that is taking steps back or stagnant. Absent of that, we have to be very aggressive in the trade market and to be aggressive in the trade market you need to have players in your roster to make it worth while. And just in case, yes we can afford Kevin Durant if we overspend on someone like Middleton.
Again, all of this assumes, of course, that a player of Middleton's calibre would make us significantly better.
To use your example, why would Durant join us if Middleton does not live up to the potential? Why join a team with some crippling multi-year deals on their payroll? You could just as well argue that a team with a lot of cap flexibility might be more interesting to Durant, as it would allow us to surround him with an even better supporting cast, and his presence would make it even easier to put all that cap space to good use.
None of this tells me why it's "pivotal" to use our cap space before 2016. It's just an opinion based on a bunch of questionable assumptions.
But here's the kicker: If Durant does not want to sign with us (I know, right?), we're then stuck with a bunch of ok players taking up our cap space, and a limited ability to improve through trades and FA. In this...unlikely case, all we did was rob us of our Plan B (or C).
It's a gamble at best, but a careless and negligent use of our best asset at worst.
You said it yourself, you're not convinced that Middleton is the guy you actually want. You mentioned other names you would prefer over him several times. Well, I agree, but these guys are not available, right now.
So, instead of waiting until they become available, you suggest we use the best asset we have to acquire these guys when they do become available, for lesser talent which may or may not make us more interesting to the first group.
I have to agree with Roy, it's a stupefying line of argumentation.