Hi Ron, welcome to Geekstogo!
All-in-one PC's definitely have their disadvantages. Yes
, in laptops and all-in-one's the components are tightly packed together, so ventilation is not as good as a regular desktop/tower computer. This would mean less room for fans, and less room for that hot air to leave the compartment area.
Heat is the number 1 killer of computer components. The hotter the system runs, the quicker it dies. Also, with an AIO, if your computer screen goes out, you are minus an entire computer, whereas on a regular desktop you would just have to replace the monitor. I don't know if the lifespan is completely cut in half, but in surely is shortened. The guy at the computer store was probably just making an example, not being completely literal.
There are desktops that are fairly small, and some can be layed on their side with a monitor sitting on top. If you are looking to conserve space, you may want to look into a desktop like that.
And BTW: You may want to edit your post and remove your email address. Bots will pick up on that and SPAM your email address. Email addresses are not allowed to be posted on the forums for that and other reasons.
Let me know if you have any questions.