EDIT: Fooled by a necropost yet again. WTF?
It's not about frugality, but about efficiency in getting the most happiness for your resources. Frugality is definitely a big part of that, but it's not the point.
To start off, depending on where you go, geoarbitrage can make travel cheaper than staying home or especially domestic travel. Staying in Thailand or Ecuador for a few months can be significantly cheaper than what you would have spent staying home in a upper-income country, even with the flight. Also, traveling long-term forces you to pare back the stuff in your life.
Even if you don't save money with geoarbitrage or reducing your possessions, you generally get a better bang for your buck spending on experiences than on stuff. Maybe we'd be happier retiring a few years earlier to live within a small radius of home, but I don't think that's true of everybody. Travel gives you new experiences. It also connects you with new people, and human connection is one of the biggest drivers of happiness. Another thing, I don't really believe that travel will save the world, but I do see a lot of misconceptions people have about the rest of the world if they've never been outside their own culture.
If you don't care to travel, though, that's great. I'm not saying everybody should. It does take a lot of resources, and maybe I'm actually a little jealous if it doesn't have any allure for you. I do think it's worth it for those of us that do really enjoy it.
Of course your happiness needs to be balanced with other people's. Flying does have a big environmental impact. I don't know how to address that except to say that I'm willing to sacrifice in other things that are equal in their environmental impact. My lunch today is completely plant-based. Meat has a big environmental impact. I'm willing to reduce my driving significantly. Actually, spending time in countries with more well-rounded transportation makes it much easier for me to reduce my impact when it comes to day-to-day transportation and regional travel.
For 1. if you think about new places to see you often tend to think of faraway places, yet how many places right around the corner have you been? I have seen more places in faraway Laos than in next-door Germany, while the latter could be done with a much lower footprint. There's plenty of countries within a days bus/train ride away that I haven't been or just to the capital, which I keep forgetting when thinking about 'new places'. This is probably the case for you too, so start identifying the nearby places that attract tourists that you haven't been.
I hear this a lot, but the point of travel for me is to go somewhere significantly different from my hometown. That may be easy to accomplish with regional travel if you live in a small country, but in a big country like the US, regional travel doesn't really give you the same new environment. Don't get me wrong; I love traveling within the US. There's so much to see, but Laos gives way more unique experiences than California if you're from Alabama. Even then, for reference, Los Angeles is further from my house than Istanbul is from Paris.
For 2. it's funny how people think they learn about different cultures when being in a place for a couple days/weeks. Think about your last international trip and ask yourself with how many locals you actually interacted; not talking about waiters, bus drivers, guides etc, but only actual people. This number is often remarkably low unless you were visiting friends or using Couchsurfing. At the same time, I know many folks living in places full of cultural diversity who never bother to look at the cultures of their own neighbours or coworkers. So identify what international exposure is there in your city/workplace and start getting to know cultures there.
True, but that's more an argument for traveling more slowly, not against travel in general.