Modern was originally defined as "every card from 8th Edition forward." The promise was a format with a stable card pool that wouldn't rotate, with cards that had at some point been Standard-legal. Modern Horizons broke that promise.
What Modern Horizons sets do
Modern Horizons sets are designed for Modern directly. They never appear in Standard. They're tested against the existing Modern card pool and pushed into the format on release. They're also expensive, supplemental products with mythic-rare staples that can dominate tournament play immediately.
The first Modern Horizons set was a curiosity. The second was a transformation. By the third, Modern was unrecognizable to anyone who'd stopped playing in 2018.
The shift
Pre-Modern-Horizons Modern was characterized by:
- Linear archetypes. Burn, Tron, Affinity, Storm, each tried to do one thing well.
- Predictable mana. Decks ran a small number of stable mana fixers.
- Slow inflation. New cards entered the format gradually, through Standard.
Post-Modern-Horizons Modern looks like:
- Toolbox decks.
Wrenn and Six,Solitude,Force of Negation. Every color has efficient answers. - Ubiquitous fast mana. Modern is now a turn-three format more often than not.
- Power-creep cycles. Each MH release reshuffles the tier list.
The trade-off
Wizards isn't pretending this is an accident. Modern Horizons sets sell. They're popular. They keep Modern interesting in years where Standard would otherwise contribute little. The argument for the strategy is straightforward: a healthy non-rotating format needs new cards somehow, and pulling them through Standard is too slow.
Modern is no longer a format you settle into. It's a format you keep up with.
The argument against is just as straightforward: a format you have to keep buying into stops being the cheap, accessible alternative to Standard, and starts being its own ongoing financial commitment. Whether that's a fair trade depends on whether you started playing Modern because it was the stable format or in spite of it.