Personally, I think that they should revert the entire repair system to the pre-1.8 one and have Mending work the same way that renaming used to - then not only do you actually need to mine more diamonds to repair your gear you still need to use anvils and cannot make max-enchanted gear if you want to keep it forever or repair it easily (for example, a diamond pickaxe with Efficiency V and Unbreaking III costs 33 levels (1032 XP) to repair with a new pickaxe; if you add Fortune III, the cost explodes to 37 levels (1406 XP) for a single diamond per repair - 4 of which are required to fully repair it for over 5 times the XP cost per use).
Even with this cost (0.9 XP per use) I was able to sustainably use such a pickaxe for all mining while caving. Similarly, a Sharpness V, Knockback II, Unbreaking III diamond sword costs 40 levels to repair with a new sword, one level too much, although you can use a slightly damaged sword to lower the cost to 38-39 levels and still get a full repair due to the anvil bonus, or use 1-2 individual diamonds for 29-35 levels, and either way it gives you far more XP than you need to repair it (6248 uses at 2 hits per mob * 5 XP per mob = 15620 XP; the surplus can go to repairing armor or other items).
In fact, I even implemented this in a mod, with some tweaks to costs so that Mending items cost about the same as before while non-Mending items are cheaper (since I made Mending cost 8 levels and each repair increases the cost by 2 levels an item which would be too expensive to repair if you put Mending on it can still get up to 4 repairs), as well as adding more expensive items in part to make more use of the XP that I get.
Also, either way the majority of players use XP farms to top off their Mending gear - I get the impression that very few players are able to get as much XP during "normal" gameplay as I do (of course, they likely use tools to mainly mine stone and other building blocks, not mostly ore) - I certainly do not think that you should have to resort to farms in order to maintain your gear; even with the XP that I get I've calculated that I need at least Unbreaking II on my pickaxe in order to keep up if I used 6 Mending items at once (in a recent world I mined an average of 4375 blocks and gained 5308 XP per play session spent caving. With the maximum of 6 Mending items being held or worn at once you need 3281 XP, assuming you pick it all up while holding the pickaxe. The cost without Unbreaking would be 13125 XP; Unbreaking I 6563 and Unbreaking II 4375. While I do not use that many items at once (3x armor + one of several held items) XP isn't evenly distributed; my pickaxe mainly gets XP from mining while my sword mainly gets XP from combat, either way I had no trouble at all keeping all items in full repair when I played with 1.9+).
Another advantage of the old system is that you can store XP in your XP bar until you need to spend it; it doesn't matter whether the items I use while caving all give me XP and similarly the items which I do not use while caving are repaired using surplus XP that I have in my XP bar when I return (I could also bring the item with me while caving so I could repair it then but I never have needed to do this). Conversely, death has a harsher penalty; what happens if you need to repair an expensive item soon but die and lose most of your levels? Likewise, the changes to enchanting costs in 1.8 were done for this reason; it costs more to reach level 30 the first time but much less to get back after spending 3 levels, such that after 3 enchantments you've spent less XP overall.
Excellent points, make mending your tools tough work again.