Huh? No you would never want to use "all" routers for a single flow anyway. Each router just needs to make a deterministic selection for each packet. The alternative to a hash based scheme would be per packet load balancing which is practically never used b/c it gives you TCP packet reordering.
>"if that router has the same number of downstream links that the first tier router had, it will send all packets it receives to the same link, ignoring other links. Which is a terrible trade-off for avoiding OOO packets"
No it would not be a terrible trade off. Optimizing for maximum link utilization only matters if you have congestion in your network and even then ECMP is congestion agnostic. In reality your leaf network has less downstream link that it has upstream. A common topology is 4x4x2 where each leaf node has two downstream links to two ToR switches.
mcpherrinm's example is worse than what I think you're suggesting. Because a given second-tier router will only receive packets that hash to a specific set of values, if that router has the same number of downstream links that the first tier router had, it will send all packets it receives to the same link, ignoring other links. Which is a terrible trade-off for avoiding OOO packets.
The more reasonable trade-off is giving up on utilizing all routers for a single flow, to avoid OOO packets.