Will common motherboards ever have very many NVMe drive slots?
Currently M.2 NVMe drives are the best drives you can get as far as performance goes, with no superior drive interconnect really on the horizon as far as I know. However, they have a practical drawback, which is that they're only available in a limited range of sizes (especially if you want cost effective sizes) and common garden variety desktop motherboards only have two or even one M.2 slots. My current home machine has two M.2 NVMe slots; my office workstation has only one and I had to get a PCIe M.2 adapter to get a mirrored pair of NVMe drives into it. An interesting question, especially for me, is if this is ever going to change and the number of NVMe drives you can connect to common desktop motherboards rises toward the level that SATA currently has (where you get at least four SATA ports on anything you really want to use).
Unfortunately I suspect that motherboards probably won't raise the number of M.2 NVMe slots they offer, even if NVMe becomes more popular and completely cost competitive with (good) SATA SSDs. The lesser reason for this is that there are only so many PCIe lanes to go around and decent M.2 NVMe slots need four per slot. However, CPU vendors can more or less raise the PCIe lane count if they want to and they already have higher counts on higher end CPUs. At this point I suspect that PCIe lane count on desktop CPUs is mostly a matter of market segmentation, like Intel's approach to ECC memory.
The bigger issue for significantly more M.2 slots on motherboards is probably the sheer space that they consume in the currently common setup of horizontal mounting on the motherboard. The common M.2 NVMe form factor is M.2 2280, which is 22 millimeters (2.2 cm, almost an inch) wide and 80 millimeters long (8 cm, just over 3 inches). Add in some extra length and a bit of extra width for the M.2 socket itself, and that's a reasonably decent chunk of motherboard space that you can't put any substantial electronics on (since they have to be short enough to not hit the M.2 NVMe, and they won't have much cooling underneath it). For scale, an x16 PCIe physical slot is apparently a bit over 80 mm long (and the PCIe card itself will generally extend further). You can get ATX motherboards with three M.2 slots (somewhat to my surprise), but I don't think there's any with four. The easiest way to fit four M.2 2280 slots into a motherboard is probably with an x16 PCIe slot and adapter card.
(People will sell you M.2 extender cables but I don't know if they're actually valid and proper under the PCIe specifications, even in theory, or how long they can be. In theory there's also U.2; in practice I doubt U.2 NVMe drives will make a comeback for various reasons. Any solution needs to be compatible with garden variety M.2 drives that can be sold in quantity and mostly mounted in normal motherboard M.2 slots.)
Whether this matters for most people is an open question (and an important one, since buyer demand will drive what desktop motherboard vendors do). SATA HDs are probably going continue to be the bulk storage medium of choice, and people who don't mirror their storage are probably only willing to get so many M.2 NVMe drives. And M.2 NVMe drive capacity will probably keep going up, which drives down the need for a bunch of M.2 drives. On the optimistic side, we're already up to top end gaming focused desktop motherboards with three M.2 slots, so that's probably going to keep on being available and maybe become more common in motherboards that are less high-end.