A 40 gallon water heater is never large enough to accommodate a Jacuzzi type tub. Typically, when a builder includes a Jacuzzi tub in a new house, they install at least a 60 gallon water heater. That is a standard construction design.

Correct answer is not always "no". It depends upon several factors.

Perhaps someone wants a bigger size tank because their usage cannot be met adequately with a smaller tank before the "hot" water becomes "cold".
Such as: Jacuzzi type tub cannot be filled with hot enough water? People need to take showers in succession or parallel? Running out of hot water for several things operating at the same time? Tired of the water cooling off during a shower.

So they decide to get a bigger tank to alleviate some of those issues. If they pick a tank >50 gallons, then federal regulations kick in that mandate a more efficient tank with increased insulation and using a heat pump system. The cost for that tank is much higher than a tankless heater (double or more). It is also a lot bigger and might not fit in an existing location.

So in that case, the answer is clearly "yes"; a tankless is the better choice since initial cost would be significantly lower.

I was just responding to the OP's simple question which was "will I save money by replacing my 40 gallon tank type water heater with a tankless water heater?" He didn't mention anything about a Jacuzzi tub. The answer to the OP 's question is "no" because he will never recover the upfront cost to retrofit a tankless water heater into the house. Obviously, if you want to apply additional factors that were not mentioned, you could get a different answer.

We have a conventional 40 gallon gas hot water heater. We have never run out of hot water. Two consecutive showers (mine at least 20 minutes; hubby 5 minutes) and run the washing machine at the same time and still have plenty of hot water. We don't use the tub for baths so that may be a problem if we did.

Not correct. There are some proposed standards that could take place in the future but you can buy an electric hot water heater today that is greater than 50 gallons and doesn't use a heat pump.