Why allowing Germany to retain Seydlitz would widen naval arms control

What changes if Britain allows Germany to retain Seydlitz after the First World War?

Germany gains a dangerous future replacement entitlement, prompting Britain to widen and tighten the naval treaty system.

Versailles limited Germany's retained ships and controlled what could replace them. If Germany kept Seydlitz, it could later claim a replacement for a true battlecruiser rather than for an obsolete coast-defense battleship. Britain would press to include Germany and other outside powers such as Brazil and Turkey in the broader naval treaty system. An old ship's future replacement rights could matter more than its immediate fighting value.