It would destroy confidence in Bitcoin and that is enough. If you are talking about a hard fork say so. No need to beat around the bush.

The rising transaction fees are a problem. It is an effective limit on growth in the long term. Small transactions are already impractical. People will turn to Bitcoin Cash eventually. A 51% attack on Bitcoin isn't really necessary. Bitcoin will become weighted down by it's own increasing transaction fees. Fine for buying a Tesla, but anything less than $200 is just plain stupid. Might as well use your Mastercard. It will save you money.

Bitcoin is over rated.