> Or a 20 for asking the eagles for help might be that they bring you half-way, as the eagles wouldn't want to get too close to Mt. Doom anyway.
I'd also add that ideally (if the DM is fully in the spirit of how this can be played for maximum creative storytelling potential), asking/negotiating for the assistance of the Eagles wouldn't be a single quick dice roll, but rather a complex, possibly hour-long session of courtly intrigue, diplomacy, and politicking, featuring many dice rolls, a lot of carefully chosen words, and a lot of favor-trading and maybe even intimidation. Ideally, getting straight to Mt. Doom via the Eagles should really be achievable, but not with just a single roll of a 20 - rather I imagine it would perhaps involve a concerted and creative effort by the whole party during at least an hour of playing time.
And then, there will be numerous dice checks to survive Mordor's anti-air assets (including probably cool eye beam lasers from the Eye of Sauron tower) and to accurately land the ring in the volcano despite buffeting wind. If any of those checks fail and the ring falls into inert dirt, the party would probably have to quickly send a commando team into Mordor to rapidly finish delivering the payload to target before Sauron's mages arrive in overwhelming force. Would honestly be more fun to play through one of these semi-failure disaster scenarios than an easy win!
Even if / when the party defeats Sauron, they don't have to stop there. For example, they could set their sights on investigating and stopping the reason for the waning of magic from the world, or any (ideally noble, or at least villainously entertaining) goal of their choosing. They could even e.g. research and create dimensional travel magic and hop to a totally different setting such as that of Star Wars.