The economic rules of an American sports league are set by a contract between owners and players that is written to end. That expiry is a feature of the design.
The agreement covers everything about money
Salary caps, revenue splits, minimum and maximum contracts, roster limits, free agency rules and the draft all sit inside the collective agreement rather than the league constitution.
Because they are bargained, none of them can be changed unilaterally by the league during the term. A rule that looks like league policy is often a negotiated term.
This is why apparently sporting decisions, such as eligibility for a draft, are settled at a bargaining table.
Fixed terms exist because conditions change
Media revenue, franchise values and the international market move over a decade, and a permanent agreement would lock in a split that stops matching reality.
An expiry date forces both sides to revisit the division of revenue against current conditions rather than the conditions when it was written.
Terms are usually long enough to provide stability for planning and short enough to track the broadcast cycle they depend on.
Opt-outs are the real deadline
Most agreements include mutual opt-out clauses allowing either party to end the deal early at a specified point.
In practice the opt-out date functions as the true expiry, since a side that expects better terms will exercise it.
Negotiations therefore begin well before the nominal end, and the calendar of a labour cycle is driven by opt-out notice periods.
Expiry gives each side leverage
Without an end date, neither party can apply pressure. The possibility of a stoppage is what makes concessions rational.
Players face lost salary and short careers; owners face lost revenue and damaged broadcast relationships. Both costs are what make agreement likely.
Deals are frequently reached close to a deadline for exactly this reason, because the cost of failure only becomes concrete as it approaches.
Expiry ripples into contracts and cap planning
Teams write player contracts that extend beyond the agreement's end, so those deals include provisions for how they are treated if terms change.
Front offices avoid long commitments across an uncertain expiry, and a bunching of short contracts before a bargaining year is a recognisable pattern.
The agreement's calendar therefore shapes roster construction years in advance, which is a governance effect rather than a sporting one.

