~28 days is a relatively standard unbonding period and provides the level of economic security needed and reduces the token velocity adequately (is there a way to quantify this beyond an opinion?) You see similar numbers on Polkadot/Substrate-based chains, Cosmos and many other chains.
However, I feel this is also punitive to first-time delegators and given the high level of distribution for GRT, the demographic spectrum is huge and would benefit from some leniency here. Cosmos has an instant redelegation feature that assists in this situation (keep in mind that Cosmos delegators are subject to slashing risk and that plays a major part in making the instant redelegation feature viable):
Technical read on Cosmos Instant Redelegation: https://docs.cosmos.network/master/modules/staking/01_state.html#redelegation
Easier read:
Cosmos Re-delegation
The 21-day lockup period for those who un-bond their stake in the Cosmos network does not inhibit someone who is already delegating their ATOM to change validators on the fly. They can instantly begin earning rewards with the validator they switch to. However, this feature is different from un-bonding, waiting for 21 days and not earning rewards over that time period and then re-delegating. This is to discourage stickiness and consolidation around validators giving penalty-free options to delegators who want to make a switch.
In this case, within the Lunie app interface, you can choose a different validator than whom you are already delegating to and redirect any amount of your staking balance from one validator to another. This process can only be done once every 21 days.
Source: https://lunie.io/guides/how-cosmos-staking-works/
If protocol-viable, something like instant redelegation would address delegator’s initial regrets around their first delegation and also reduce stickiness and indexer consolidation.