It would be a big change sure, but not huge. The change would only have to do with final build and games that went over 35-40 minutes when the most fed person has a full build with current item slots. Other than that every game would be exactly the same as it is now. The end result should actually be that there would be less 50 + minute games because the team in the lead would have more time to be in the lead.
Most big changes always feel daunting or scary and then time goes by and you adjust and would one day talk about how in your day you remembered boots taking up one of your item slots.
I'm not claiming it would be a positive change but it's something I would definitely like to see experimented with to find out.