Your "Isabella" is the blue sister variety of the original "Red Isabella" with red berries. You have chosen a plant that is not only delicious (a delicate strawberry note!), but also relatively resistant to fungus. So, it really is a shock if such a plant is infected with fungus. Having said that, exactly the same thing happened to me recently with the "Arkadia" type; this is a divine type, which produces grapes in your garden at home that are just like the large, Italian table grapes that we see in the shops. I have now cut "Arkadia" back completely and I'm in the process of replacing it with "Pleven ustojcivij".
The background to the whole story is that resistance to fungus in table grapes is always limited. You go for years without any problems, then the weather can cause a "fungus year" with a level of infestation that is so high that even an otherwise loyal garden type like "Arkadia" or "Isabella" succumbs. This doesn't mean that the type is poor; it is down to the underlying conditions.
In practical terms, you now have a choice of two possible strategies. On the one hand, you can proceed as you have suggested: building the vine up again from scratch. On the other hand, you can plant a new table grape type, in the hope that it performs better in that position and if there is another high level of infestation.
In the first case, I advise treating the plant and its surroundings regularly with a fungicide as a priority. You have to include the rear wall and so on because they are covered with millions of spores. I would also recommend treating the surrounding area for the second case — that is, replanting.
Overall, despite the setbacks, I won't be banning my various table grapes from the garden; it's an issue that has now become far too exciting for that. It's a similar story to strawberries: if you aren't successful with one type, then choose a different one.