Benefit&#x27;s obvious - getting rid of legacy cruft. It has to be done someday, or by 2030 we&#x27;ll end up with few kilobyte-long UA strings.<p>Cost: bajillion man-hours of CEOs, companies and committees trying to negotiate that they all do this at once, then about 10 minutes of developer time to actually code this into reality.<p>Then some sites would be broken, but old stuff eventually rotting and dying is nothing new on the web.  Unfortunately, no idea about the numbers, but I suppose it would be low. If someone has the actual statistics on how much sites become practically unusable without a legacy User-Agent header, that&#x27;d be great.