without restricting metamorph and sphere, you’ll have 11 sphere effects. With 12 lock pieces eldrazi can 5-0 a league (montolio). Those lock pieces that montolio used, they are definitely much less effective than shops lock pieces. Plus, it is workshops verses eldrazi temples. That is why shops are decently right now (not counting Oath)
With 5 lock pieces and various other taxing effects, shops will still be very very strong. The state of the game will be much more interactive. Let me give an example. The shops guy plays a sphere, found another one and played it, and then metamorphed another sphere. The blue player’s brainstorm costs 4 and plays maybe 2 spells max before being beaten by artifact creatures. Even with only one sphere brainstorm costs 3. Hollow one and two anglers can win for dredge, 5 sphere effects can win for dredge.
This is why we need tangle wire and thorn restricted. Tangle wire into tangle wire? 4 copies of thorn that could end your game for two mana? 9 sphere effects that are just about a must counter. Are you kidding me? 9 sphere effect cannot be balanced for the game. Just because there's 8 sphere effects does not mean that they shouldn't be restricted.