- Have devastating effects on the fee market
Since there's more space in each block, this space is now less valuable, fees might not be zero, but they'll be lead down to the minimum and competitiveness for fast transactions will be dead.
I don't know how it would change the fee market.. There hasn't been any scarcity in blocks before so fees would probably stay quite similar to what they are today. If adoption continues to increase, the total of collected fees will be higher as there would be more transactions per block.
- Bring nodes offline and make pools/miners less reliable
The change would increase bandwidth consumption as well as HD space required to run a full node. There are many nodes that run on non-optimized hosts, if they suddenly start sucking up more of the bandwidth and HD space it's safe to assume that many will go offline. Also, pools running on weak networks would be more prone to orphaned blocks because of the blocks being harder to propagate.
One possibility is that increase in adoption and advancement of technology will balance that or even make number of nodes increase. We can't know for sure.
What I know is that letting blocks to saturate will make the fees rise rise and will make bitcoin less inclusive. It will be less appealing/useless for many markets/use cases thus stiffle the growth. Everyday users will find it less useful and investors don't want to invest in niche product with no growth potential.
If you're not so sure about that, then lets try to keep 1 MB limit for few years just to see what happens. It would be an interesting experiment and surely would not kill bitcoin...just push it back few years at worst.