I am not sure about cheap ARM devices but I am using an old Haswell i5-4670 and it is more than enough. So it won't be issue later.
Also, when you are talking about consumer NAS, the real problem is that any low-end systems can saturate the gigabit network (100MB/s) very easily so investing on extra resources for ZFS doesn't make difference. At least a 10Gbe network (which is beyond the average consumer) is required to actually make it useful.
I repurposed a micro Dell Optiplex 3060 with 8GB RAM and two external HDDs totalling 9TB of space. The CPU is an i3. The whole thing takes less space than a book.
I have lz4 enabled and the gigabit link is almost completely saturated when transferring: 119 MB/s out of the total theoretical 125.
No ZIL, no L2ARC devices are attached. That thing is _flying_ as a home NAS.
I am going to assume in three to four years time this wouldn't be a problem? I mean a 16nm Quad Core ARM Cortex SoC are only $15.
Unfortunately no consumer NAS are implementing ZFS. ( TrueNAS offering isn't really consumer NAS )