Not very silly no, but since you asked for reasons why not
. As others have said you would definitely need to get a friend/companion for him or her if there aren't other horses living in the field.
Heavy horses can be a PITA to buy tack and rugs for (even XL horse size will often not fit and they need very wide gulleted saddles, very hard to find and fit), and obviously bigger stuff costs more. They also wouldn't necessarily fit easily into a standard trailer/towing set up, certainly not if you are travelling two side by side!
They eat a lot, would your budget stretch to generous hay and possibly some hard feed over winter, is the field big enough to accommodate such a big horse and still allow you to divide in two in winter and rest a part or would it end up picked bare?
Not all heavy horses fit the gentle giant stereotype, some do of course but shires in particular are known for actually often being quite hot and sensitive. I think as with all horses it comes down to good consistent handling and schooling but obviously with such a big animal it becomes more of a problem particularly if they forget their ground manners and start getting bargy or pulling....
They were never bred to be a riding horse so can have some confirmation issues, they tend to do fine as hacks but if you or your DDs ever wanted to get serious about schooling for dressage or do jumping etc you may find you struggle!
If you are hankering for a low maintenance horse my honest advice would be buy a steady cob or cob x , they still have the hairy 'look' but that bit smaller and easier to handle, and a better chance of becoming a nice mother daughter share in future. But if a heavy is what your heart desires then go for it!