My point still stands though. It's also kind of up to you what you want to achieve. There are for example plenty of bejeweled like games and mahjong games that simply aren't able to be cleared, but that doesn't mean it ruins the game.
Now with a game like this, the more you clear, the more options you get in terms of space you can traverse. If I, for example, clear 2 blocks that are connected to each other from the side or top, I create a lot more options to connect blocks to and the larger this gap gets, the more options you will get, making it almost impossible to not be able to clear it if you have enough starting options, which is very controllable by you
Of course situations can arise where you aren't able to finish it. It also depends on how many variations of blocks you are using. The more variations of blocks, the harder it gets to connect them.
I also find it hard to believe, that if a field is truly randomly generated, you will always be able to clear it. Very high chances sure, but not 100% as you can never anticipate the move a player is going to make next, unless the field is made up in such a way that only one way is possible.
I'm not saying this is end all, there might be a way to prevent deadlocks, but I personally think it's just a very high chance of clearing due to how the mechanics work