Sandboxing does not necessarily mean Status Quo encounters.
My 5e online game is an open-world sandbox, but it's not entirely a 'status quo' sandbox - I fairly often have encounters tailored to party level, partly because that's what the available wandering monster tables are designed for. So eg when the party decided to go on a quest to destroy the Black Sun Gate I was using a lot of ca EL 10-12 encounters, matching party level.
You can have sandboxes that are entirely open world and entirely tailored, like Elder Scrolls III, and this is a way to get around the level-up issue. Generally the 'zoned' approach of low and high CR/level zones works better for immersion - eg the area around the Black Sun Gate was one of powerful magical energies.
Currently my party are 12th-14th level and command their own fleet; with 3 warships and 200 warriors (down from 280 before the last battle) - significant encounters are with enemy armies! It's still entirely open-world; they can take their forces whereever they want and decide who to fight, who to seek to recruit, etc.
Read more: http://www.enworld.org/forum/showthread.php?482109-D-amp-d-is-not-a-good-sandbox/page5#ixzz44aSEdih4