adoption vs replacement

OP's idea is not adoption, its replacement. and to give the short answer, goodluck running down that path before you learnt to walk

EG users class ebay as being main stream adopted by consumers. that does not mean there is no alternative/choice.

pepsi and coca-cola work side by side
walmart and 711
mcdonalds and burger king
dollar euro pound yen bitcoin

they can all work side by side. its strange to think that investment people will only invest in one thing because someone on a forum wants to irradicate all other investments. its strange to think that consumers will only spend in one currency because someone on a forum wants to irradicate all other currencies.

especially when he has not done the maths to realise that bitcoin doesnt need to have 100% population grab with 100% life savings for a currency to function.

especially when the OP has no understanding of bitcoin to know that bitcoin is not just about using in a shop.

This is a great distinction!

To the great examples listed for the adoption of Paypal or Credit Cards, you're right in that they didn't require that no other form of payment be accepted.

So perhaps a better question is, for best adoption should retailers say "We accept BTC, and also cash, credit, paypal, etc" or should they just list BTC at the end of the existing familiar list of payment options?

No it won't retailers accept multiples forms of payment already. Bitcoin will be just another form they accept. The customer may chose. This is a nice way of letting the customer chose how he/she wishes to pay for their goods or services. Even a modest adoption would give Bitcoin a lot of traction!

hopefully one day some retailers will just accept cash or bitcoin and bypass credit cards or bank cards
just like a lot of restaurants in my area are cash only

Replacing real cash with bitcoin could be something that is way further into the future than many may think. I personally don't see it happening any time soon to be honest and even if it did there would have to be so many regulations put onto it, it could become ridiculous. Until businesses big and small knew they were safe to accept bitcoin I don't see many taking the plunge.

No, not by any means.   Full adoption would mean nearly every retailer offering to accept BTC right along side fiat.

IMHO - we are more likely to see BTC supplement fiat and credit cards. The vast majority of the public are debt serfs and require credit to run their lives, paying interest to their masters.

The best way for brick and mortar stores to start accepting BTC and spread the use of BTC would be for them to follow the offer that some online stores have offered, 10% discount on purchases with BTC. Cash is cash, any b&m store hopes that all their customers would use cash to avoid the fees associated with accepting credit cards payments, but when accepting BTC, those fees petty much disappear. Might be best for them to start at 10% for initial interest into the idea of paying with BTC, then after so much time, possibly lower the discount to 5%.

No, it won't. Those things will happen around roughly the same time though. When Bitcoin reaches 80%+ market saturation globally, fiat will soon cease to exist (have no value).

I cannot speak with certainty about the fate of gold or altcoins, but I'd cautiously argue that the outlook is poor for both. Bitcoin is the only place I truly feel my money is safe right now.