Yes, I've used some rewards flights (F SYD-LAX v.v) and now a DFW trip in J to help retain my status. Indeed flights over the next few weeks should get me over the line which is a nice little perk.
Of course, there is a much smaller intersection of high status (WP+) with PC/PC+ membership - not that high status pax don't earn PC through spend, but more that since PC was set up as the "Frequent Buyers" status I imagine the bigger number of members might be more points hoarders than higher status pax. Obviously QF knows the break down.
So yeah, one could make 1400 on reward flights IF they can get them opened, and fly a bunch through the year, but it would still be a bit. eg: SYD-LAX in F earned me, iirc, 81 SC, so 160(ish) for a return, with DSC 320. Do 4 of those and you're basically there. You earn about the same with SYD/MEL-DFW in J (which is more achieveable than LAX in F). LHR of course is a bigger earn so you could probably just about get there with say 3 r/t plus some other bits an bobs. Is it practical and realistic? for some it probably is. Most certainly niche.
Could certainly make Silver and maybe Gold probably fairly easily if one was motivated and could take advantage of DSC.
My own SC earn from rewards this membership year, for example, is still only around 15% of my total (but DSC has been involved with revenue tix). Helpful, but not a sealer.