You can just use your Bitcoin for transactions over $100 and for less then $100 you use doge or eth or ltc or something. Many more places have now accepted altcoins and have a drop down menu to select them.
This doesn't help unless you have pre-bought various alt-coins in advance. Changing them on the fly with something like Changelly would cost more in conversion fees than just making the bitcoin transaction and paying a higher transaction fee. You also run in to the problem of different vendors accepting different alt-coins. What if you aren't holding the right one at the right time?
On the whole, the much simpler and easier option is to start using Lightning Network. More and more vendors are now accepting LN, you can open a channel when fees are low and then spend whenever you need to, instantly and cheaply, regardless of what the current mempool status is.
If you planning and making purchase in other altcoins you just plan ahead and buy some and use them as you need. Most people should have various coins if they serious about cryptocurrency.
LN has been spoken about for months and it's still not an option. If people don't hold eth then I don't even know what to say since you need it for token transactions.
Ideally you should be able to use any currency with similar fees. So like you say you don't have to hold the other coins.