The biggest problem with HLS is that the HLS vendors still want to pretend it's "C++ / OpenCL / whatever to gates". What you get is pretending that there is no such concept of a clock even though you know it is always there and you care about it, and the language you are really writing consists mostly of all the crazy pragmas that you have to sprinkle over everything. It ends up failing on both counts: it isn't C++ to gates, and it is an exceedingly difficult HDL to use because it tries to hide the clock from you always even when you really need to do something with it (e.g., a handshake).
A weak spot of high-end commercial HLS tools (Catapult, Stratus) is in interfacing with the rest of the hardware world, and how the clock is handled (SystemC, you handle it yourself) or kind of vaguely (Catapult's ac_channel). Getting HLS to deal with pipeline scheduling is great, but sometimes you want to break through and do something with the clock. Want to write a memory DMA in HLS? Talk AXI? Build a NoC in HLS? Build even something like a CPU in HLS? Interface with "legacy" RTL blocks, whether combinational or straight pipeline or with ready/valid interfaces or whatever? These things are sort of/just feasible at present with these commercial HLS tools, but very very hard (I've tried it).
If they want to stick with it, I think C++11 could provide a superior type-safe metaprogramming facility for building hardware (compared to the extremely primitive metaprogramming and lack of type safety notions in SystemVerilog) or generators such as Chisel or the hand-written Perl/Python/TCL/whatever ones in use at most companies, but sometimes you need to break down and do something with the clock or interface with things that care about a clock, much in the same way that one would put inline asm statements in code. I want to do that, but not have to deal with the clock 95% of the time when I don't really need to, which is where the generators fail (let the tool determine the schedule most of the time). HLS needs to sit between the two: not a generator (glorified RTL), but not "pretend you write untimed C++ all the time" (not hardware at all).