It's shocking the amount of hate directed at the idea of new mechanics because of what it could mean for old work. Let's consider some things:
1. This Probably Wont Break Your Farms.
The game does not constantly update all blocks. Water mechanics are changing, but I seriously doubt the block updates are. Chances are your water streams with torches wont break until you personally update them. Yes, some farms will have redstone that will trigger that, but not most. For the most part any existing use of it should be fine. You'll just have to get inventive with new ones, which might be more fun because
2. These New Mechanics Mean a Lot of Cool Mechanical Potential
We'll be able to make an item float or sink, presumably even in running water. With fences, trapdoors, etc, we'll be able to filter items in a stream. We can already place items on either side or in the middle with blocks like ender chests and panes. So we'll effectively have a water stream with 3 channels, we can raise or lower, and block any combination of them. You can have a water stream feed middle items forward and items on the left down. I'm sure there's a lot of clever things people will come up with.
In short water is becoming more complex and so water streams can behave in more complex ways.
3. You Are Not the Only Player
Realistically most players are children who will not at all be affected. Even out side of that, I would bet most players don't make heavy use of the things we'll see break. It's the silent majority. We've all been on servers. I only play on servers with adults who are competent at the game, and still most of them would not be affected negatively.
You know what all those people will get?
4. Better Water Aesthetics.
The ability to place plants, fences, iron bars, slabs, etc in water is HUGE for building. Water falls can go down gentle slopes for once and look good. We can build under water with something other than solid blocks. We can have ponds with more variation than just sand, dirt or gravel. It does so much for people who like building in this game. Are there people who'd rather not have to fix their water streams more than allow so many people have a better game?
5. Mojang Has Shown Maintaining Game Breaking Farms is Not A Priority
They've broken farms and exploits before. Remember booster carts? People used to have huge lengths of tracks with carts to bug you out into going faster. Then they added booster rails and broke the carts. We got over it.
When someone asked about optimizing exp orbs to fix performance at xp farms, one of the devs agreed with the idea "Maybe if you're breaking the game you should expect lag".
While definitely core for a lot of us and a big part of the game's appeal, maintaining game breaking farms should never handicap game development. We will find new water streams (and as I mentioned, probably better ones). We'll have our game breaking stuff again. But the maintenance of your sugar reed farm you copied from the internet just should not take priority of things like more complex water mechanics to make better water ways, or better buildings in water.
Maybe having more building materials than you know what to do with, turning your game into creative mode, isn't important. And guess what, it wont break anyways, so you still get to.